I'd agree to go with UA

* E+ when you reach elite level is invaluable.
* More flights to SFO
* Can get miles on US, and AA's flights to LGA are on undesirable RJs
* Terminal C better than Terminal B


For the last 12 months, the market shares at Logan are actually DL 19%, AA 16%, US 14%, but these will probably change since DL has cut flights (esp. to Florida) in bankruptcy and US has America West now.