I'm stuck between the two universities too!! If this is any help here's my pros list of both ahah:
St Andrews:
Pros: Higher UK Rank, I like their traditions, I prefer the course and the flexibility of their structure, highest student satisfaction levels in the UK
Cons: Apparently the nightlife is lacking (no actual clubs), Quite deserted location wise (I'm from London so It'll definitely be a culture shock), Lack of diversity (could be said for both), the nice accommodations don't have en suites lol, Americans.
Durham:
Pros: Collegiate System, 3-year course opposed to 4, better location, decent nightlife from what I've heard, Russel Group (if that even matters), Pretty town.
Cons: Lower Rank, can be said for a lot of unis but Oxbridge reject central, Durham culture seems lowkey toxic apparently but I'm honestly not too fussed.
I think St Andrews is like the best university for IR so that may help your decision, I'm planning on doing Econ and IR there. If you think of anything else let me know I have no idea which one I prefer tbh. Also, I'm fully aware of how trivial a few things this list are they're just what came to mind lmao