I'm at Wolfson so take my comments for what they are worth:
In terms of location, St. Cross is located on St. Giles which makes it one of the more central colleges. Wolfson is about 20-25 mins walk from St. Cross. That said, Wolfson has based on an extremely picturesque site by the Cherwell. I've got a room that overlooks our gardens and the Cherwell but, conversely, the noise from the geese and the punters on the river can be a bit of a distraction at times
From my experience, the community at St. Cross is a somewhat more fragmented than Wolfson. I suspect this may be due to the college's size, and the fact that many of the grad students there don't live on site or in college accommodation. What I have noticed is there appears to be a much closer grad student-fellow community at St. Cross than at Wolfson. Again, I suspect this may be due to the fact that St. Cross is somewhat smaller than wolfson re: numbers.
In terms of facilities, St. Cross has quite a nice bar and common room, but neither were heavily frequented on the occasions I visited there. The common rooms at Wolfson are ok, but they are usually only busy after meals, before formals, and on the Sunday afternoon AMREF coffee shops. To be honest, the cellar bar is the real social centre of Wolfson rather than the common room. Library wise, I think Wolfson's book stock is a lot bigger but, being a college full of humanities students, the science collections aren't particularly amazing. The librarian at Wolfson has ordered pretty much anything I have asked for. I can't speak personally about the St. Cross library and the librarian's propensity for responding positively to book requests, but I do know someone who was able to get a book request fulfilled without too many problems.
Foodwise, I haven't eaten at St. Cross at lunch, but I'm told it's pretty good. Lunch and dinner at Wolfson can be a bit hit-and-miss. I think the quality of the food at formal halls is roughly the same, although I think Wolfson has a slight edge. I've only eaten a formal at St. Cross once though, so maybe I chose a bad night.
I don't have any experience of accommodation at St. Cross, but I get the impression they have a much smaller stock than Wolfson. Many people at Wolfson live in college for 1-2 years, and some people manage to end up living in college for the duration of their courses, in the case of MPhils and DPhils. Accommodation isn't that cheap though, but there are moves afoot to try to reduce the usual annual increase over the next few years.
With regards to clubs and societies, some of these are shared between Wolfson and St. Cross; the boat club and football are the two clubs that spring to mind.
With regards to college ents, Wolfson is definitely the more active of the two colleges. There are also plans to do more joint Wolfson-St. Cross events, but nothing much happened this year on this front. Maybe next year will be better
St. Crossers are invited to the Wolfson sports and societies fair at the end of week 0, so there should be some opportunity to mixing here.