Choose Downing! *isn't biased in any way at all*
It's definitely one of the best colleges for law, usually has a fairly large intake and as has been said, we've got Professor Virgo
The atmosphere is excellent as well; there's usually about 120 people in each year, so it's small enough that you can know pretty much everybody in your year, whilst large enough that if you really don't get on with somebody you don't have to know them!
The bar is cheap-ish, it's cheaper than your average (English) pub for sure, but there are cheaper student bars (eg Emma) It's also kinda small, but that just means it gets really crowded after formals. There's an extra room beneath the bar as well, for ents and the like, as well as for special events. So in general, socially Downing's pretty good. Head down to the bar any evening and you're bound to see someone you know, and probably make some new friends while you're at it!
We do have a choir, and some music facilities, but I don't know much about them so I can't tell you very much! I do know we have a few pianos that are available to students if they get permission from the head organ scholar, and I also know of at least one Downing-based band, so it's definitely do-able.
First year accommodation is pretty awesome; a lot of it is hotel-standard, since the college makes its money by renting the rooms to conference guests during the holidays. And most of these are ensuite; some even have baths...
It's certainly not the closest to the centre of town, it's maybe a ten-minute walk to Sainsburys, but it's pretty close. However, it's close enough that going into town is no problem, and it's far enough away that we don't get incessantly pestered by tourists, so the location is actually pretty good! However, I don't think it's especially close to law lectures, I'm afraid. But being Cambridge, "not especially close" generally means no more than a twenty-minute walk!
As for the other two, I can't pretend to know much about either of them, but We'd rather be at Oxford than St John's...
Hope that helped! (Now come to Downing