I am considering getting a card mostly for lounge access in US and Europe since I was just bumped from Gold to Silver status on Air France/KLM despite 150K+ miles.

I have looked at Priority Pass, Amex Platinum, and several others but cannot decide what the most advantageous would be for me. I travel mostly between US and Europe (France, Germany).

Any advice?

Are you close to or 100% loyal to a particular carrier or alliance (I am assuming you fly more than just AF/KLM based on tier)? That will factor in, since specific lounge membership and/or access is included on some cards (annual fees apply, naturally).

e.g.
Delta Reserve (SkyClub access)
Continental Presidential Plus (Presidents Club membership)
American - multiple cards (Admirals Club membership discount)
etc.

If you mainly fly international on AF/KL, maybe you would want to pay for a Delta SkyClubs membership because it has reciprocity at KLM lounges in Amsterdam and I believe may provide reciprocal access to some AF lounges-- I haven't test tested AF recently because I have been ST E+ for a number of years, so it might be good to research AF lounge accessibility. In any case, it sounds like the lounges you want to use most are the AF/KL ones that are simply not accessible through credit card memberships. Thus, my idea is that a U.S. lounge membership purchased from a Skyteam carrier may be better for your needs. By the way, if you use KL alot, it might be worth checking with them, as I think they had some type of purchaseable status (at least at one time) that provided lounge access in AMS. Let us know if you find an interesting credit card lounge combination that meets your needs.

IMHO, the best value for money relative to lounge access is to get the Ameriprise World Elite Mastercard. The fee is $150/yr and gives you Priority Pass.

A Diner's card is handy, depending on where you travel, I pay next to nothing in yearly fees. Check their web site for lounge availablilty.

Diner's US no longer accpets applications.