Right now there's not a huge difference in seniority between the bases. It seems to vary bid by bid. Anyone with more than 5 months seniority (who isn't seat locked to the E190) can hold any east base. PHL and DCA will get you to a block marginally faster.
As of the February bid, guys hired mid to late 2011 are holding secondary blocks on the Airbus.
If you are looking at it from the perspective of an AA pilot, I'd suspect that by the time we are able to bid each other's bases a lot will have changed including East/West seniority integration throwing Phoenix into the mix. I would expect Phoenix to become the senior base, since I don't forecast it growing and the Westies will come away with significant seniority no matter how the whole Nic fiasco plays out, simply based upon the number of pilots hired after them.
I don't expect DCA to shrink based upon statements Parker and Horton have made but anything is possible.