AA codeshare is useful on LCIRCx fares (it earns mileage crediting to AA), but otherwise not useful for D or A fares. Upgrades are never available on codeshares, you must book the real metal if that is a consideration. Finally most airlines treat codeshares as 2nd class at some point, so if you don't care you might be better off booking the real metal instead of the codeshare.