Just going by airports alone, I really like Vegas (I go there at least once a year). It's nicer, we've had easier times going through security there, and the walk doesn't seem as long, although you do have to catch a tram from the departure terminal to the outer terminals. I also prefer driving in and out of the Vegas airport area than I do LA. But with either airport, you'll need to catch a shuttle to the car rental -- again, I prefer Vegas for this too.
While I wouldn't say there is really anything bad about LAX, given an equal choice, I would rather go through Vegas.
Keep what dletz said in mind though, driving to Vegas from LA on a Friday is really the PITS! As is driving to LA from Vegas on Sunday. Traffic can get REALLY bad!