Thank you all for great advice. I will probably go with AA. Since I don't have many miles, the ability to book an offseason award for only 40K miles with AA will be useful to me until I accumulate more miles (and hopefully fly J some day!) and their North American stopover option sounds like a great feature. AA doesn't let you search for partner availability online, but it's something I can live without.

The only thing is that the taxes/fees seem pretty high if I fly to Europe with a BA connecting flight. I've booked an award travel to Europe once on AA and paid a little over $200 in taxes/fees for a coach seat (BOS-VCE and then FCO-BOS return with a BA connecting flight in LHR). Is that charge considered pretty typical or high for an award? Is it similar with United? I have no reference point since it was my only award trip.