One thing to note if you are flying short routes on UA, UA will not have any type of "meal" in F on those short hops while AA will have a better food selection in F and if you are buying last minute and F is sold out, you will not get a free snack in Y on UA.

That said if you frequently have to same day change UA is a great option.

I'd take the GS status and use it when convenient, but would stick with AA for any normal flight booking.