I've flown both and found them both to be good. I've only flown UA in J class but they were better than I expected and the service was friendly and genuine.
UA must be suffering now their service from MEL is limited to a continuation 744 from SYD.
I don't understand why they didn't introduce a non-stop 772ER service from MEL-LAX which is quite poassible despite ETOPS. Under their current structure QF own the MEL-LAX route with NZ and UA offering only token competition.