I lived in both, although not strictly as the off-campus apartment was also owned by the school.Campus life
If you're in a shared 'Foreign Teacher House', you'll likely make a lot more friends than any other way. I was in such a house, and after we had CD Jr, I was very glad of it, as I couldn't really go out larging it because of the baby, but there were still lots of people around.
Very short 'commute' to work (unless you have the bad luck I had and find you're being sent to the 'branch school' and have to get up at 7.10 to catch the school bus.
No bills. This may or may not apply to an off-campus apartment. Depends on the school.
Guaranteed free internet.
You've got the entire school grounds as your hideaway at weekends. Our school was always empty on Sat/Sun and I really dug this, especially in the hot summer months, could go out and lie on the grass on the training field.
It's not necessarily smaller accommodation, though, the special family flat our school gave us was absolutely huge. You could skateboard around the front room.
You will get no privacy. I remember the school headteacher coming to see us in our upstairs flat just totally randomly and unexpected.
You might have on-campus internet, but check whether this is really your own internet or if you are sharing the school connection. If it's the latter, it will be frustratingly slow during the day and may well get switched off in the evenings. This happened to me before and they denied it for at least a year before they finally got ADSL just for the FTs' house.
Curfews as others have said. At my first place they said we had to be in by 10.30 and couldn't knock on the gate.
Luckily my mate and I found that we could go out, then come back later and just climb over, but it's hardly the most desirable thing is it?
Office hours. I know that technnically a school can have office hours whether the teacher lives in Campus or off it, but if someone's on campus, it's much easier than if they're off campus to go blundering up to their door at 8.35am because they aren't in the office yet (even though their first class is in the afternoon). The office will invariably provide little to do while in the office, though, and if you pass the time yourself (and it's blatant) they'll tell you not to do it.
You obviously get tons more privacy. Because ours was school-owned, we could still get things fixed when we wanted it, but they never, ever just turned up at our doorstep, in fact the only school people to ever visit my flat were the maintenance people
Because of that, it will be much easier if you want to invite students or groups of students for lessons.
It might be on the 9th floor or something. Schools in China are notorious cheapskates. My Dalian place was on the 7th floor (in Chinese/American, 6th floor in British).
Lonely at times. If you've got a good social life going on, then fine, but if you're like me and don't get much chance to get out, on campus living can be good, and feels a bit like being back at Uni at times.
Bills. Can add up if you aren't careful
No free internet (although you won't have any connectivity problems)