Not going to lie but a MSci sounds better than a BSc and it is a higher degree, I would probably go with Bristol, unless of course you can get into a 2nd year entry degree at St Andrews.
I go to St Andrews, its a great school, small and tight knit and you'll make plenty of friends but I'm on a 3 year course and I wouldn't know if I would want a 4th year. I think Bristol being a larger city might be a more comfortable place to spend 4 years. It totally depends on the person however, some people love St Andrews to death and others I know can't wait to leave.