I'm in these airports a lot. They are total peers for your purposes when things go smoothly. LAX has nicer clubs and PHX is a bit easier gate-to-gate.

But when things go to hell, LAX is an nightmare of unbelievable proportions. In my experience things go to hell less often in PHX, things are generally more on time, and things recover more quickly when there are problems.

I'd go through PHX unless you need to take a shower during the layover. You're going to need those nice clubs at LAX when things go south, if you are lucky enough to avoid a gate hold.