I'm also SEA-based, and I'm faced with the same question as the OP. I've asked a somewhat similar question elsewhere on the board. I got mixed advices, and so far I'm choosing to credit all the miles from AS/AA/DL to AS (even though I flew DL>>AS>AA last year).
Even though the OP has made his/her choice, I'd like to continue to read more people's input on this. I'm sticking with AS for now, but I'm still on the fence about AS vs DL miles.