Which particular fact about Jizya are you disputing eh? Land is considered wealth, and any non-Muslim with land would have to pay Zakaat on the land AS WELL as Kharaj (land tax), not just sadqaa - so it appears that Kennedy is mistaken on this point. See below for a full treatment on this point - with some references to other historians too - and note it says the land tax was the same for muslims and nonMuslims - therefore the comparison does come down to Jizya. (Good attempt though, shame you didn't do a bit more research though)
The point is that I have quoted Kennedy speaking about Jizya - where he confirms the facts below - I'll leave it to you to dig out the quote - you managed to find the above quote (which doesn't say how much Jizya was) - so I'll leave it to you to dig out what Kennedy specifically says about Jizya.
The facts are:
1. Jizya is a tax (some argued it was more than this)
2. It is a simple tax, payable by only able bodied and employed men, and is fixed amount per annum.
3. the tax ranges from 12 chickens a year, to 48 chickens a year (or 36g of silver to 144g of silver)
4. The tax is minimal in absolute terms - as poverty was defined as anyone who could not SAVE more than 17 chickens a month (so you the tax was less than 6% of savings - and as it was fixed, the more you earned, the lower the tax burden)
5. Muslims paid taxes which varied with wealth
6. Arnold (and others) have quoted the rates
7. The arithmetic (as well as the conclusions by Arnold et al) all show that the moderate Jizya was less than Muslim taxes (its a mathematical result of the rates and forms of taxation)
8. Some Christian tribes elected to serve in the military and were exempt from Jizya, some Muslims wanted to be exempt from military service and had to pay tax to compensate
However, here is an interesting quote - which you can look up on Google books. Please read, analyse and report back on what it is saying was the cause of the decrease in revenue in the Umayyad period when people converted:
"How were non-Muslims taxed and how did they hope to benefit by accepting Islam? What did 'Umar do in attempting to reconcile the rights of the new Muslims (mawali) with the revenue needs of the
government?...The difficulties arise because the sources rarely supply the detailed and precise information we need, or if they do, the information may relate to only a limited area of the problem or a part
of the Umayyad territories...Even the lengthy and quite detailed so-called 'Fiscal Rescript of 'Umar II' is subject to these remarks. It has generally been accepted as a copy of a document sent by 'Umar to his
governors giving them precise instructions on certain questions concerning taxation and the rights of non-Muslims to accept Islam, but it is clear from the attempt of H. A. R. Gibb to explicate it that it
leaves a number of issues in doubt, and, in spite of the general acceptance of its authenticity, its ascription to 'Umar as a whole can only be impressionistic and open to question. Like almost all of our Umayyad 'documents', it survives only as part of a later literary text - there is no archive and we do not have the document itself, if there was one.
If we take first the question of the taxation of non-Muslims and Muslims in the Umayyad state and the advantages to be hoped for from acceptance of Islam, it would be fairly easy, in theory at any rate, to provide an answer if the classical Muslim fiscal system had existed from the beginning. In this classical system Muslims specifically pay a religious tax, the zakat, which is levied at different rates on different types of poverty and wealth. Non-Muslims specifically pay a poll tax, the jizya, payable on the person of each non-Muslim, both as a sign of their inferior status in the Islamic state and as a return for the protection which this state offers them. Thirdly there is the kharaj. This is a tax payable equally by Muslims and non-Muslims on land which is liable for it, generally land which was conquered and became the property of the Muslim state but which was left under the cultivation of those who had worked it before its conquest, subject to a tax which was to be gathered for the benefit of the Muslims as a whole [but what about for the benefit of non-Muslims?]. When this land changed hands it remained liable for the kharaj, no matter what was the religion of its proprietor. In this system, then, there is likely to be some incentive for the adoption of Islam by the non-Muslims, so long as the financial burden of the zakat was less than that of the jizya, but the amount lost
to the treasury by the conversion of an individual was unlikely to be significant for the finances of the state.
In the Umayya period, though, this system (admittedly more complex in practice) did not yet fully exist, and, largely as a result of the work of D. C. Dennett, it is apparent that we have to speak of fiscal systems rather than of one system covering the whole empire. Nevertheless, Dennett seems to have shown that, in spite of the diversity, there was at the level of the tax payer over most of the empire a dual system of poll tax and land tax, and a reasonably precise terminology to distinguish them. Acceptance of Islam should in theory have brought relief from the poll tax in most areas, but the land tax would continue to be payable so long as the convert remained on his land...Full fiscal benefit would be gained both by conversion and by abandonment of the land, but it was abandonment of the land which was crucial since this freed a man from the land tax and removed him from the clutches of the tax official. It was this abandonment of the land which caused the
problems for the Umayyads in both Egypt and Iraq, and it was against this background that al-Hajjaj's measures against the mawali makes sense. Almost everywhere widespread acceptance of Islam would lead to a decline of revenue, either for the government where it collected taxes directly (as in Iraq and Egypt) or for the local non-Arab rulers and notables charged with levying it and paying the government an agreed amount (as in Khurasan). To prevent this decline in revenue the government or the local notables, as the case may be, either tried to prevent conversion to Islam or took no account of it when collecting taxes...
It would be unwise to attempt to be too specific about 'Umar II's response to this situation. Beyond a general acceptance that there should be no distinction in Islam between Arab and non-Arab and that there should be no obstacles to the acceptance of Islam by non-Arabs, it is difficult to pick out specific measures which we can be sure about.
Some reports say that he forbade the acquisition of tax paying land by the Muslims after the year 100 AH (AD 718-719). This seems to indicate that Muslims did not expect to pay tax on their lands and as more and more tax-paying land passed into Muslim hands, either through its acquisition by Arabs from its previous non-Arab cultivators or by the conversion of its non-Arab cultivators, so the government was deprived of a vital source of taxation. There is a possible allusion to this measure in the 'Fiscal Rescript', but the wording is rather vague and there is no mention of any date. It is reported that in Khurasan a deputation led by the pious Abu 'l-Sayda' complained to 'Umar that the governor al-Jarrah b. 'Abd Allah al-Hakami was imposing circumcision as a test on would-be converts who were flocking to Islam in response to 'Umar's insistence that the mawali should be freed from their kharaj tax and receive proper pay in the army. 'Umar's response was to depose the
governor, forbid the circumcision test while insisting that 'God sent Muhammad to call men to Islam, not as a circumciser', and to demand that the non-Arabs entering Islam should receive equality of treatment with
the Arabs.
But whatever 'Umar did or did not do, it is clear that he did not provide a permanent solution to the problem, for after him we continue to hear of non-Arab Muslims being subjected to what they saw as unrighteous taxation, of consequent discontent and even revolt, and of renewed attempts by individual governors to recognize their rights. The problem seems to have been especially acute in the east where it caused
constant trouble in the territories east of Khurasan, while in Khurasan itself we hear that in 738 thousands of Muslims were still taxed while many non-Muslims were getting off scot free. Dennett argued that the dual system of land tax and poll tax, with remission of poll tax but not necessarily of land tax for those who accepted Islam, should have remedied the grievances of the mawali without injuring the interests of the state. But this works only if it is assumed that the system was respected, and then only in those areas where it operated...When, therefore, as seems to have happened from time to time, an attempt was made to encourage the local population to accept Islam by promising remission of taxes, it was the local rulers and notables who came under pressure because they were the ones responsible for paying the agreed sum to the Arab governor" (78-81).
Hawting, G. R. "The First Dynasty of Islam: The Umayyad Caliphate AD 661-750. New York: Routledge, 2000.
http://books.google.com/books?id=03kB6yjps4IC&pg=PA78&lpg=PA78&dq=%22non%20muslims%22+taxed+less+than+muslims&source=web&ots=UMgoZ3tRAH&sig=RI0g66QDa-vybQuZlSnqeOglBS4#PPA80,M1Cheers,
Shafique