Edit: Oops - answered the wrong question initially.

For Premium Economy, VX/AS Main Cabin Select beats UA by a bit because the in-flight entertainment is a bit better (since you can have pretty much as much as you want), and I think that the complementary food and alcohol is a bit better. Having said that, UA has much more schedule density so they'll have more opportunity to take care of you in IRROPS. VX/AS also only has Main Cabin Select in bulkheads and exit rows, which I generally try to avoid, so I generally prefer the Economy Plus seating locations on UA.