Well, hang on a wee second here.

I agree that AA's is an excellent program, and with your flying pattern (100K miles mainly in premium cabins) you could easily hit AA Executive Platinum, Oneworld Emerald (the highest tier.) With that status comes 8 SWUs (on AA metal) and unlimited "free" upgrades on AA North American services, so certainly not to be poo-pooed.

BUT, your description of your intended likely travel patterns (a lot of international J segments ex-SAN, likely on BA metal) and your questions about upgrading do raise a point.

BA's "MFU" - miles for upgrade - works very well, and might save a lot of money going forward. Flying out of SAN, you'd purchase a Premium Economy ticket in the cheapest bucket (T) then use BA Avios for upgrading to business, or, buy a J ticket and MFU into F (which I don't think is offered on SAN-LHR, but you get the idea.) AA miles can't be used for upgrades on BA metal unless the underlying ticket is full-fare (Y, B, W etc.) which of course is usually more expensive than buying a business ticket outright.

AA elites (Plat or EXP) also are not allowed to use AA lounges on intra-North American flights, while non-AA Sapphires and Emeralds are, so if you were a BA elite you could use Admiral's Clubs in North America regardless of destination or class of service flown. Partner elites also get priority boarding and other perks on AA flights.

One achieves BA status through the accumulation of "tier points" - so many points per flight according to class of service and distance flown. To achieve BA Silver (= Oneworld Sapphire = AA Plat) you need to accumulate 600 tier points. A return trip in J from JFK to LHR would earn 140 TP in each direction; a return trip in AA F from SAN to JFK would earn 210 each way, so with the one return trip you'd have earned 280 + 420 = 700 TP, bingo. (Note you also need 4 BA-metal flights to actually make Silver, so you'd need to buy a cheapo return from London to someplace close - Paris, Edinburgh, whatever - to fulfill that criteria.)

So practically speaking you could do one of your trips crediting to BA, make Silver, get your card and use the AA lounges, then do the Plat Challenge the next time and use AA as your main mileage earner going forward. Of course you'd want to credit some to BA (using your 100% bonus) to build up your mileage there for use with MFUs going forward.

One other thing with the BA program is the existence of family/household accounts, so members of the same household can pool their mileage, a good thing if you travel a lot but other travel a little. Don't know if this applies in your case, but worth mentioning.

Oh, and AA recently changed the rules on the Plat challenge so that now you can use elite points (remember, it's 10K elite-qualifying points, not miles) gained on AA, BA or IB-designated flights, not just AA metal or codeshares.