A FlyerTalk Posting Legend
Join Date: Sep 2009
Location: Minneapolis: DL DM charter 2.3MM
Programs: A3*Gold, SPG Plat, HyattDiamond, MarriottPP, LHW exAccess, ICI, Raffles Amb, NW PE MM, TWA Gold MM
Posts: 102,617
Often you get additional elite benefits when flying the carrier of your FF program, so it can be good to credit miles to the airline you actually fly.
It sounds like part of the OP's tradeoff is some chance of domestic upgrades when flying UA as a UA elite versus getting lounge access as a nonUA *Gold when flying UA domestic.