Just some tips about US air carriers and airports, for those not familiar:
Generally speaking, try to avoid O'Hare (Chicago.) O'Hare is notorious for delays. The airport is overburdened, and they are at risk of weather delay at all times of year. Also, going that far west will add hours to your airtime, though it will do nice things for your air mileage account.
Boston and Detroit can have a lot of weather problems in winter; they often get "socked-in" (planes held on the ground) due to snow. If you are travelling in winter and really want to avoid the possibility of snow delays, pick either Atlanta or Miami. (Snow is possible in ATL, but very rare. They do sometimes get ice.)
To look at on-time statistics for both airlines and airports operating in the US, use this gov't site:
http://www.transtats.bts.gov/OT_Delay/OT_DelayCause1.asp
To check seat pitch, width, etc., you can use
www.seatguru.com
(They list all US carriers that fly overseas, and also BA and Virgin.)
US carriers are a lot more lenient about baggage limits than UK carriers are. Our trick is to put your small but heavy items (books, toiletries, power adapters, chocolate, etc.) into your cabin baggage, as US carriers do not weigh cabin baggage. (There technically is a weight limit, but unless your bag cracks the counter when you set it down, they won't enforce it.)
Checked bags are weighed & the limits enforced, but again, the limits are more generous than you will find with UK carriers. If you're tight on weight, tuck an extra soft but sturdy holdall into the checked bag; it is much cheaper to check an extra bag than an overweight one, so you can shift the excess to the holdall and check that, too. You can avoid the extra charge by never planning to take the maximum number of allowed bags for your party; the extra holdall will still be within limits and thus will not cost you anything more.