I was forced to move to AA or UA last year when I switched jobs. DL from MSP is way too expensive to be within policy on a regular basis. I have been very happy with UA. They don't handle irrops as well as DL, and there seem to be a few more of them, but the phone support is superior with every call I make. It isn't dependent on lucky on call centers. I get as many upgrades as gold on UA that I did as Platinum on DL. One of the best benefits I've come to realize is the partner network. Having Star Gold while travelling international means access to great lounges. That is important to me. UA is slowly getting their act together, by copying DL on a lot of things. Hopefully they don't copy the horrible "partner network" that is SkyTeam.
All that said, I say go to AA. I've heard some rumblings from AA elites, but you get that from any airline. Don't worry about being "hub captive". I say that is a term that only applies if you are too lazy for an extra flight and thus fewer elite miles.