I agree that OP is probably best served by DL due to the benefits from the Cent card. And if OP is positioned to use DPJ to move to DM, then even better.

However, I think there's argument for using AS as your FFP even if you don't live in/near SEA and never fly AS, *if* your main focus is on mileage earning and redemption.

You won't get many airport/inflight benefits (I think AS MVPs are below DL FOs on the UG list), but many folks on FT argue that the ability to continue earning based on distance and the generally favorable redemption rates/availability on a variety of partners is quite attractive.

I certainly credit all my AA flights to AS, and on other occasions where I'm not on DL, I also check to see if I can credit to AS before registering for yet another FFP scheme. AS can be a nice catchall/secondary program due to its partners.