Oxford Maths+cs or Cambridge Maths

It really does depend on personal preference I think - there isn't really an objective answer to which is best. First thing to consider, probably, is whether you'd prefer to do a joint honours course or not.
Then there's the location. Cambridge and Oxford are pretty similar in that regard, but Cambridge is definitely busier and livelier, so that should factor in. Then again, the collegiate systems make this a little less important.
As for the admissions exams, you're right about tmua being much easier than STEP, but you're also right about Cambridge inviting more applicants to interview. Presumably Oxford looks for very high scores on tmua.
A bit anecdotal, but to provide my experience, 2 kids in my year applied to Oxford maths, and 2 applied to Cambridge (myself included). Both Oxford applicants were rejected without getting an interview, and both me and the other Cambridge applicant did get interviews and we're waiting to see if we get offers - but, to be completely honest, there's a high chance that we don't get an offer, or that we miss our STEP requirements. It might be worth thinking about whether you expect to do well in your interview - if you do, perhaps Cambridge is the better choice given that you'll have a better chance to show off. If, on the other hand, you're very confident about tmua, Oxford might be preferable.
Regardless, don't make any decisions until you attend open days. I was convinced Id be applying to Oxford, until I actually visited them both and completely changed my mind.
Sorry for the essay lol but I hope it's somewhat helpful. How's year 12 been going so far?

In what way is Cambridge 'livelier'? I totally disagree ...

I am year 12 doing maths, fm, comp sci and econ and I am stuck in choosing whether to apply for oxford maths+cs or cambridge maths, probably will take both to a masters degree level. I haven't been to an open day at either yet but just really would like to know which I should do if anybody has any advice. I do a lot of maths in my spare time, I also do a lot of programming and robotics, although I believe that I prefer maths with application in computing and programming. In the future I want to go into computational mathematics / mathematical finance. I feel like the TMUA for oxford is significantly easier than STEP for cambridge, however Cambridge offer far more places and the filter is usually STEP, also a lot more people get interviewed in the first place.

Idk, as someone who has spent a fair bit of time in both towns, Cambridge always just seems consistently busier, louder, etc. There seems to be more people out and about, the market seems more active. The mood also seems a bit more upbeat but of course that's very subjective, hence why I advised them to visit for themselves.

I would say the opposite so, yes, it depends on when you go and where you go. For example, in Oxford there's always live music in Cornmarket Street free to just stop and listen.

I am year 12 doing maths, fm, comp sci and econ and I am stuck in choosing whether to apply for oxford maths+cs or cambridge maths, probably will take both to a masters degree level. I haven't been to an open day at either yet but just really would like to know which I should do if anybody has any advice. I do a lot of maths in my spare time, I also do a lot of programming and robotics, although I believe that I prefer maths with application in computing and programming. In the future I want to go into computational mathematics / mathematical finance. I feel like the TMUA for oxford is significantly easier than STEP for cambridge, however Cambridge offer far more places and the filter is usually STEP, also a lot more people get interviewed in the first place.