Hey, I'm living in Grosvenor Street Building this year so I'll try and answer some of your questions. Here's a quick rundown on all 4 halls in the Grosvenor Site:
Grosvenor Place
Largest out of all four halls, with about 450 people. You aren't in a flat as such, but in corridors. They are 6 floors high, separated into blocks A to H. So each floor has 8 different blocks, and each section has 10-12 rooms, which share 2 shower rooms and 2 toilet rooms. Each section/flat thing has a new kitchen this summer too, which are nice, if a tad small. I like GP a lot, the only bad thing I think is that if you have a swipe card for the whole site, anyone can literally get right outside your room door and into your kitchen, as the flats arent separated off by separate keys or anything. The other 3 halls are in self-contained flats. GP is lively, sociable, most people in each block seem to know each other, there have been a fair few parties in some of the kitchens etc. The halls itself were built in the 70s but have been refurbished etc, they dont appear dated.
Grosvenor Street Building
Obviously I'm biased, but I think I made the right decision in choosing this hall over the others. its the newest hall, built 1994, and has good decor etc. ROoms are separated into flats of 8 people, sharing 2 shower rooms and 2 toilets. I've never had to wait for the loo or a shower since I've been here. The kitchens are large with a cooking area and a separate living area, with a tv and chairs to sit around and watch it. GP doesnt really have a lot of 'chilling space' or a big table to eat at like GSB does. There's always a lot going on here, but I think it depends on the group u get in with - some people you hardly ever see but others are always around. There are three flats on each floor, and it's 4 stores high. Its divided in half into smokers and non smokers (so 12 flats in each half). My room is a decent size, I can fit everything in. Obviously its not huge or anything, but then none of them are.
Ronson Hall
The smallest out of all 4 halls, only around 100 people I think. But it's nice, built 89/90, large-ish kitchen. Breezeblock walls in the rooms (the other halls have plaster) but thats not really a big deal. Looks wicked from the outside. Everyone I know living there is really happy. 8 rooms in a flat again, with 2 showers and 2 toilets in each flat. There are 13 flats in all.
Bowden Court
My least favourite, it looks really dated and grotty from the outside. I know someone that moved from Bowden to GSB because her room was a built grotty. Built in the 70s, in need of a refurb i think! Still good tho, very lively. The kitchens are big too. Same set up, 8 rooms, 2 showers, 2 loos.
Overall I think I'd be happy to live in any of them, but I hope i've helped u enogh to decide which one to choose. The security for all 4 is tight, swipe cards for all the main entrances and to the individual halls as well. Apart from GP, no one can get outside your room unless they have swiped in at least two places, and have a key for the flat.
Hope this helps
Jenna