Be careful with United. I bought a lifetime membership way back when it was the Continental Presidents club. Anytime access flying or not. United took over and kept everthing the same until a year or two ago. Then they imposed a requirement to actually be flying Untied Airlines. That screwed me since Untied does not go too many places out of Seattle (they had a lot more flights out of SEA years ago).
The Alaska club will allow you in regardless of what Airline you are flying, you simply must be flying, as you have to show a boarding pass. WIth the Alaska Plus membership you can get into the AA lounges when flying Alaska or American.