For an US-EU traveler, losing status in UA means losing access to E+. In general, it seems like it best options have been to get status on the airlines you fly the most frequently, since you can get upgrades and other bonuses from them that you can't often get as a partner elite. Having said that, if your flying is infrequent enough to not warrant elite status on that airline, then all sorts of other things come into play.

I'd looked at AC and JK at one point when nearly all of my flights were on US. The ability to get to *G was faster with both of those, but I wouldn't have been able to upgrade any of my domestic US flights. As it was I stuck with US, managed to just squeak by *G, and barely flew in coach for domestic travel for over two years (even on the 8F 757). If I'd gone with AC or JK I'd have gotten lounge access within the US but been flying in coach. Personally, I'll take domestic F upgrades over domestic lounge access, but really each person has to make their own decision.

There's a thread from 2003 where Patron compared the programs as they existed then:

http://www.flyertalk.com/forum/showthread.php?t=183976

I know they've changed since, but the comparison is still interesting.