Well, I don't know much about Trinity - the rooms I've seen have all been pretty nice, but I've not seen many! They have some off-site stuff on Grange Road, which is not really far away, but as almost all of John's is on site, I guess you could count that as a difference.
John's I have more experience of, as my ex was there. I would say that the truth lies somewhere between the two extremes you've heard. Many of the rooms are very spacious, almost all are on site (I think a handful of second years have to live out, and that's not far away), and there's a reasonable number of gyp rooms/bathrooms for the ones which aren't en-suite. En-suites definitely exist, though you most likely wouldn't get one to yourself until 3rd year. The sharing thing is sort-of true - most people will share in 2nd year at least, but it's not actually sharing a room - it's a set of two rooms, so you have your own bedroom/study but one of you will have to walk through the other's to get out usually. As for renovations, it's not all brand spanking new, but I wouldn't say it was falling apart. Cripps is hideous, yes, but the rooms themselves always seemed ok. Rents are pretty high though, compared with many other colleges.