Welcome to FT!

I would definently join the UA Mileage Plus FFP, since your LAS-LAX flight is on Ted. With US/HP also a major presence in LAS, when you fly them you can credit those miles to your UA account. And with partner purchases (for grocery shopping) at Von's/Safeway, you can earn more miles to your UA account!

When I lived in the LAS area (actually IFP but flew mostly from LAS), I earned many FF miles with UA just doing my normal shopping! Now living on the east cost, I earn many FF miles with CO doing my normal grocery shopping at Shop Rite!

When you fly, join the FFP of whoever you fly. On my many flights out of LAS, I flew (and joined the FFPs of) UA, US, DL, NW and CO. When redeeming awards, UA may not have one, but DL might. Or CO may not have one, but US might.