Having thought about making the jump myself (once I drop from AA EXP), there is one other thing about AA that you should know from SEA versus LA.
Depending on where you want to go, premium award seats on AA are tough to get out of Seattle. Specifically, connecting to an international gateway. You can find J or F space from the gateway to where you're going (I'm talking specifically Europe), but the SEA-gateway leg often doesn't have availability.
I don't know if it's the same for MVPG's.
After you have AA lifetime PLT in hand, I'd make the switch. One of the other key benefits of MVPG versus AA Plat is the lack of change fee.
Good luck