Oh yah, Oxford College ratings of rooms really don't mean much, just ask these things as they are the only ones worth paying for IMO.
1. Ensuit or not? (i.e. your own bathroom)
2. Private Kitchen?
3. Windows (I go insane w/o natural light)
4. Is it next to a laundry room/boiler/anything which makes loud noises (happened to a friend at St. Ants. drove him mad)
I don't think 1 & 2 are options at StX
St.X is a very neat, tidy college in my exerperiance, so you shouldn't worry about filth with them. Only one girl had problems 2 years ago, with a phantom smell, and the college bent over backwards to help her. All there buildings are new so they haven't developed Oxfilth yet.
If you live out and want a nice place w/o breaking the bank, live in Summertown and accept the long commute or get good at biking. Jericho and Cowley are mega popular w/undergrads and thus have only expensive or otherwise bad places left. EVEN IF THEY LOOK GOOD ONLINE IT IS A LIE. Undergrads que for days in november to get the good places in Jericho/Cowley. The good stuff is gone by now. Trust me, its true. Summertown is quiet, too quiet for undergrads, and has a nice set of shops, pubs, restraunts, and grocery stores. It's knid of its own little town. Bus passes are not worth it, though, if you live there, accept you will cycle. If that is not OK, live in college for a term, then try and move out if you hate it.