My another penny...
For UA or AA, they were among the oldest programs and they know how to treat elites.
But, for UA, at least mid and low tier elites can still have UDUs. For AA, mid and low tier elites need to be on YB for that (although you do not have a budget).
Second, for major airlines, UA on time rate is unbeatable - GAs know how to push the planes away from the gate with the minimal time.
I will not consider anything from Delta or VX.
Delta's program is known as SkyPeso. Then you know how people value the program.
VX does not have enough partners.