Jim, Nerva’s measure you cite above would have only applied to Jewish Christians, and the numbers of Jews who convert after the late 1st century is relatively insignificant. It was against the law for Gentiles to be Christians; they were not given any tax benefits. So tax considerations cannot really be a major reason for the spread of Christianity in the 3rd century.
The jizya is probably better viewed as an attempt to decapitalize non-Muslims rather than an effort to dodge taxes on the part of Muslims.
Charles Adams’s history of taxation is probably the best out there, although ancient taxation is not its exclusve focus,