I don't know anything about MBA courses, but I can tell you that Oxford is a wonderful city, albeit one with a very different pace of life to London. There's plenty to do and see, but the atmosphere is a little less... hectic?
If your decision is based more on the course than on the location, I guess you need to work out which course has the best reputation (I assume there's a league table for MBAs?) and, importantly, which one is structured in the way that sounds most appealing to you.