Sheffield med school doesn't have that many lectures, in 2nd year you only have a handful and there's none whatsoever in 3rd year and beyond. The massive year group doesn't seem like much of a problem, everyone seems to know everybody else and like you said it does mean there's a lot more societies that cater for medics. Russell Group and the age of the med school don't really matter unless you plan is to practice outside of the UK (maybe even EU?). It's a city campus university so you get the best of both worlds, there's definitely a very strong community feel here. There is the potential that you will need to stay in places like Doncaster or Barnsley (or further) during placements at Sheffield, however from what I've heard it's doable to commute from Sheffield although if you want to do so it would be advisable to get a car or live near the train station.
Sheffield is very safe as long as you know where you're going, in the 3 years I can think of maybe two incidents that have happened in the better areas (as a UoS student you'll have no problem staying in those places). As a city it's not that busy, and it only gets particularly bad on the trams and in the city centre during big events, both things which are fairly avoidable as a student, except when Ice Hockey Varsity is going on.