What would you say is the argument for considering the Quran a somewhat more reliable historical source?
Or would you regard it as equally unreliable ?
It depends what you mean by historical source. Both are nearly useless if read to correctly relay contemporary historical narratives about historical facts. The Qur'an takes pains to avoid even trying to do this, instead preferring to operate as a sort of anonymous text that is almost devoid of contemporary narrative context. The few exceptions are probably the latest and crudest interpolations, such as 33:40, where we finally get what seem to be a couple proper names (MHMD, Zayd) in the context of contemporary narrative proclamations ... also the solitary Qur'anic mention of Makkah.
But both are critically important historical sources that bear on the specific societies and beliefs at the times and places where they were composed. The Qur'an is not more historically reliable, in that sense, but it was composed much earlier than the hadith, and thus is a much better resource for deciphering the very early history of what became Islam.
A question that has been occupying me lately is this: Did the Qur'anic Kuffar Exist? At first glance, it seems to be a ridiculous question. The Qur'an is always railing against the unbelievers, hence the unbelievers existed and were seen as a specific group of people distinct from the believers. Patricia Crone has written a long article on who the "mushrikun" likely were, and takes this to be a historical question about a specific historical group of people; she thinks they are monotheists of some sort.https://www.hs.ias.edu/files/Crone_Articles/Crone_Quranic_Deities.pdf
But that is only true if we accept the picture of the Qur'an as a contemporary text delivered by Mohammed to his faithful. If we think of the Qur'an instead as a theological composite that was largely assembled *in response* to Mohammed's death, in the decades thereafter, then the people called 'unbelievers' and 'associators' are essentially theological constructs who are invoked to make a dogmatic point and help articulate a new religious community.
It has been convincingly argued (by Hawting etc.) that the Qur'anic 'polytheists' were probably monotheists, and that the later Muslim arguments about jahiliyya were ahistorical fantasies. But the more I think about it, the more I suspect it is an error to read the Qur'anic diatribes against polytheists/unbelievers as a historical description of historical conflicts that were actually happening at the time of textual composition. Rather I think the pre-existing language of monotheists struggling with kuffar was *anachronistically* adopted over time to help articulate a new religion, increasingly distinct amongst the monotheists. In this sense, there never were any kuffar, any more than there were 'sinners.' The Qur'an is not talking about actual people. You basically have early believers who are articulating what they believe, and trying to distinguish it from those who do not believe it. Not a conflict between well-defined actual groups of historical people! A theological conflict, vaguely imposed on (intentionally) vague underlying circumstances, anachronistically borrowing the language of more archaic conflicts between monotheist and pagan.