You have a difficult yet envious choice to make!
Both are top-notch operations with a fair amount of CA attrition. AWAC is a little over 1/4 the size of Expressjet, so any growth with shorten upgrade times. That said, AWAC has not been known for a quick upgrade, although most CAs I've flown with upgraded between 2.5-3 years. Both have strong ALPA leadership and good CBAs.
XJT junior base is EWR (I've heard); for AWAC you will be assigned any of our bases (DCA/PHL/ORF) in training but can probably hold wherever you want to go by the end of IOE. DCA is the junior FO lineholder base for AWAC.
If I were you, I would be a little concerned about XJT's situation with losing airplanes with CAL. US Airways would love to get rid of some 50 seaters as well, but AWAC has 9 years left in a very secure contract for 70 aircraft. If I remember correctly, we can replace our 50 seaters for 700/900s on a one-to-one basis if ATW and Airways management agree. There are lots of rumors abound regarding 10-15 more planes for Midwest, but nobody knows if that is actually going to happen, and if it does, when the airplanes will actually come on property.
Bottom line: go to whoever has domiciles you want to move to, or who has the easiset commute from where you are or want to live. From looking the contracts from both places, you'll make more $$$ as an FO at AWAC and probably slightly more as a CA at XJT.
Good luck with your decision!