Depends on what you are looking for:

SFO:
1) Larger, nicer lounge
2) Showers available
3) Better food selection
4) Better "bubbly"...slightly...
5) Usually, not as busy as LAX since LAX (in the afternoon) has several flights leaving around the same time: NRT, HKG (for now), JFK and LHR departures as well as inbound people connecting from SYD, NRT and HKG (for now).

LAX:
1) Easier to access (don't have to transfer to INTL terminal)
2) Much better UA staff people
3) Nicer view (I think in the LAX lounge)
4) Hollywood effect...And if you are into "Star Watching"....see alot more in LAX because of the JFK flights (personally, for me, this is a minus, but sometimes it can be free entertainment with the "DYKWIA" attitudes...)
5) Will offer escort down to the planes
6) Did I mention the much better UA staff there?

I vote LAX for a more stress-free experience because of the "clueless yahoos" in SFO.