I live in Santa Barbara and moved over exclusively to AA in 2002. Prior to that I split my travel between UA and AA and flew Delta occasionally. I know the AAdvantage program very well, I have a good grasp of the MP program but I don't know the Delta program well enough to give you honest advice on that one. If you want free international upgrades, I'm assuming that you will reach the highest tier and will do a few MR's if needed. I wouldn't count on the double mile promotions but I do know that UA and AA always match each other when the other announces such a program. I fly 6x/year to Europe, 6x/year to Asia and about 12 domestic trips/year on AA so here is why I'd recommend AA:
1. AA and UA have a much bigger presence on the west coast than Delta, especially LAX and if you want to fly out of SBA. I'm not sure where you will fly domestically but if you fly to JFK, the AFS is great in my opinion.
2. The only direct flight from LAX to CDG is on AF, a Delta partner, but I think you have to pay full fare if you want to upgrade. You will need to connect if you fly AA, but if you are willing to be a mileage runner, I am sure a connection doesn't bother you. You can fly LAX-LHR and use the BA F Gallery lounge or the AA arrivals lounge. The F Gallery is very nice. You will get to use the First Class lounge at LAX as an EXP traveling internationally as well as all of the other OW first class lounges when traveling internationally.
3. If you reach EXP on AA, you will receive 8 EVIP's, good for upgrades anywhere on AA metal. If you fly 4-5 times to Paris each year, you could use your EVIP's for 4 of those trips. AA offered 4 extra EVIPS last year if you hit 125,000 EQM. You can use EVIP's on any fare. UA provides 6 systemwide upgrades at 100,000 miles and I believe they are somewhat restrictive.
4. The award program is very generous at AA and there is expanded availability as an EXP.
5. The EXP agents are fantastic and I have never had an unpleasant experience with them.
6. If you plan to travel to South America, AA has a great presence there. AA is weaker in the Far East than UA. They only fly direct to NRT, PVG, and PEK (starting in April). You will need to connect through ORD for PVG or PEK but those extra miles add up! And if you can ever experience CX's first class lounge at the Wing, it is hands down the best airport lounge in my opinion. The Peninsula catered restaurant is very nice and the cabanas are incredible. And when you want to take that trip to Asia, use your miles for a first class ticket on CX!
7. You will get free domestic upgrades on all of the programs. I can't speak for the other programs but I can't remember the last time I haven't been upgraded on AA. I hear it is more competitive for those open seats in the other programs, especially out of LAX.
I recommend reading the stickys in each program's forum, there's tons of great info there to help you make a decision. I'd also check out these recent posts in the AA forum about Delta PMs switching to AA:
http://www.flyertalk.com/forum/delta...ion-delta.html
http://www.flyertalk.com/forum/ameri...ng-aa-exp.html
Good luck!