How many are in the traveling party? If the answer is two, that might lead you to the 763 or the one version of the 772 that is still 2-5-2. (Not sure where these 772's are these days, how easy it is to find them, and how reliable the type information is 5-7 months in advance.)
I also like the MCI mini-cabin on the 763. If you don't move your feet too much you can kind of pretend you're in J..
(OK, not really...but it is a bit of a more private cabin.)
Last time I flew ORD-LHR, it was a 763.
All of that said, I'm not sure I'd book based on type since it seems to change so often.