I left Delta because their SWUs are basically worthless. AA would be much better for international upgrades.
But I fly thru CLT a lot on US. Decent hub.
If intl UGs were the priority I'd go with AA.
If direct domestic were the priority, I'd go with US and ask for a sillver status...or challenge to silver. *G is basically Just lower than US silver status anyway.
You cannot reserve good seats or get any UGs with *G when flying US air.

I'm even thinking of crediting 25K of UA miles to US so I can get silver status and get some decent seat selection.