Again, not enough info. What is your objective? Free award flights in business or economy? Domestic US or international? Better comfort (i.e. UA's E+ seats) or upgrades on your international flights?

Simple accumulation of lifetime status can be done on any of the majors (though AA includes all earned miles including credit card miles ^)'

Personally I like AA for lowest tier because you can reserve exit row seat, plus the domestic sticker upgrades are cheap and often clear. UA is better if you want extra room in economy class for international flights, but awful for award redemption on partners and until recently pretty mediocre on international business class saver awards.