It's a tie for me. The CRJ200 is by far the most passenger unfriendly plane ever built. That said, US is one of the most passenger unfriendly airlines.
I suppose there are some extra frequencies on US for that route, so in case your flight gets delayed, you might have some additional options to ensure you get to LAX. IIRC, UA only has 3-4 daily flights on PHX-LAX.