Although I don't study Biomed, I've had a few lectures in the building and it's a really nice space to study in! It's on the main campus in Park Place, just a few minutes walk from the city centre, near Bute Park and minutes away from where most the students live. This makes it a really convenient location to study.
Personally, I love Cardiff - to the extent that I've even written a travel guide on the city. There is so much to see and do and it's one of the most affordable cities in the UK which is great for maximising your budget. There are excellent transport connections to London, West Wales, and Bristol Airport for any city breaks (only an hour away).
I hope that this helps a little! Both Sheffield and Cardiff are great cities so you can't really go wrong with your choice.
All the best,
Jaz - Cardiff student rep