Hi, I'm a second-year studying maths at Warwick. St Andrews was one of my options, and I did visit on an open day, so hopefully I'll be able to give a little bit of help.
There were a couple things that put me off St Andrews. First was the international league table ranking, where it's generally placed pretty poorly outside the top 200 for maths. Lots of people will say to largely ignore this because it's not a great indication of education quality or student experience, and they use a lot of factors that may not be relevant to you. Whilst this is true I do think it should be taken into account. I've read that when applying for masters / phd studies one of the things they look at is university ranking. I would guess the same is true when appling for your first job, especially if you want to work outside the UK like me.
I also wasn't too keen on the atmosphere. It felt a bit posh and stuck-up to me, with quite a lot of private schooled students. The town is quite small so it might feel a bit isolating if you don't like it there.
With regard to pure vs applied, pretty much everyone imagines themselves as more pure than applied ... but after a year or two of actual pure university maths I think lots of people tend to end up liking applied more. This was the case for me
Maybe you end up liking pure more but yeah you can't be sure at this stage.
My best advice would be to go and visit the uni if possible. I know it might not be with things how they are but I do think the feel of the place matters. Also, whilst it's obviously an important decision and you want to make the right one, I think you should follow your heart. If somewhere doesn't feel right to you, I wouldn't recommend going there. That's my advice, do take it with a grain a salt as I spent ages deciding where I wanted to go and I'm still not sure if I made the right choice with Warwick (the course is very good, and the societies are great ... but it's a LOT of work).