Welcome!

Are you looking to choose an alternate airline solely on the breadth of what a lounge membership offers?

The only thing that would jump out to me between an Admirals Club membership (via CC) and a United Club membership (via CC) is that the Star Alliance has alliance-wide recognition of paid lounge members. In other words with a United Club membership you and a guest can visit nearly any Star Alliance carrier business class lounge while flying Star Alliance. If you aren't going to get any status or flying in First/Business in Europe this could be helpful as you can visit lounges at many airports.

American will provide lounge access at select Intercontinental outstations to partner/contract lounges, but only if you are departing that airport on American. So if you're flying within Europe, there would be no lounge access. Of course if you keep the Amex Platinum you would still have Priority Pass which is pretty good outside the US.