i'm not in catered but a lot of my friends are, and i will say that the JMCC (pollock's canteen) can be quite social but i can imagine difficult to start out with. if you're looking to be forced into making friends i would not recommend chancellor's court, as the 'pantries' (kitchens) are shared between large groups of people and it kind of feels like a hotel. other accommodations may be different - if you can attend an open day you can look around some of the places then, otherwise i think you can find out how many people live in a flat/share pantries online. i'd say generally smaller flats make it easier to be friends, but it really does depend, as i'm in a flat of 5 and we've barely spoken all year (for reference i'm in murano, which is the other side of the city).
as a general rule en-suite places are much more expensive and so you need to consider if it's really important to you as the price difference is very significant, but i'd say the most important factors are location and flat size.