Sussex has loads of new accommodation? Don't worry about it being run down! A few of my friends currently go to Sussex and I visit quite often and stay over too. Every space is sociable in one way or another, as long as you've got house mates. It'd differ each year as to who you get in your accommodation, so you can't really determine which accommodation will be more friendlier than the other.
If you're looking for an en-suite and in a fairly new build, I'd suggest Northfield. You'd share a flat with around 6-8 people in Northfield. Or you can opt for a studio flat where you have your own kitchen and bathroom and private space, but one of my friends is staying in a studio flat in Lewes Court and she said it's been so hard for her to find friends, even though she goes out often. Because people tend to hang around and stick with their flatmates a lot. If you're looking for an en-suite closer to everything on campus, then I'd suggest Lewes Court phase 2, but it's one of their older blocks. The rooms in Lewes Court are quite a bit bigger than the newer builds though. Swanborough is also a good pick - it's right in the heart of the university and the buildings are new, but ofc that does come with a dearer price tag. The rooms in Swanborough seem to be the smallest too. I think I'm going to put Swanborough as my first choice though, purely because it's right outside the Jubilee Building for Business, Economics & Finance. And that's the subject area I've applied to.