I would absolutely follow the money! It is true that Oxford carries a level of prestige that few universities are able to match, but when it comes to academic work, if your institution is of a good caliber (which Aberystwyth certainly is) you'll be able to produce good work and will be well-regarded. Do not underestimate the personal prestige and connections of your supervisors, either! Not all the experts on everything are at Oxford.
I don't think over £100000 in debt is worth Oxford's reputation. Aberystwyth wants you enough that they're willing to pay for it. Oxford is happy to let you come – if
you can pay for it. As somebody else said, if you're not independently wealthy, it's a polite rejection.
I get that Oxford is known worldwide and Aberystwyth isn't, but particularly if you want an academic career Oxford's name alone is not worth it. Postgraduate research fellowships and later faculty postings are often more concerned with your research, and with the funding you can attract. For example, take
this listing from Exeter for a lecturing position. They want 'a strong record in attracting research funding, or demonstrable potential to attract such funding'. You'll have that at Aberystwyth, not at Oxford. If Aberystwyth is doing good work in your field, historians at other universities will know and will recognise your qualification and achievement.
I am an international student too so I understand the difficulty of finding funding. You got the funding: take it!
P.S. On the other hand, while you're out of the big competitions like Ertegun and Clarendon, it might be worth waiting to see if you have any chances at college funding, for example. Some of those are not announced until the summer! Don't self-fund, but think carefully about turning down the offer.