Thanks again for all the great info and advice. This was my first request for help since discovering FT recently (can't think what did I did b4 FT. Maybe I had a life...can't really remember ;-))
I'm overwhelmed. Thanks, guys!
Anyway, I still want to go with AA, that's my gut feeling, but living in Scotland, OneWorld partners to Europe are really expensive. I need to pay B or Y to qualify for any points at all with European partners, so I think I need to go with United. If only I had a US address - you guys are really fortunate with the FFP's, you know!! ;-)

Scots,

One of the biggest distinctions that I see, which only applies if you plan on doing osme significant flying or charging a lot on credit cards, is their "easy" to obtain lifetime programs. Just earn 1 million miles and get lifetime gold, 2 million gets ou lifetime platinum. you should be able to do this much quicker than 1 million actual miles on UA to get lifetime Premier Executive. AA's top level (Executive Platinum) is amazing (great service, free upgrades domestically on B fares, etc.). But if you are doing lots of A/P travel, then UA is the way to go.