Honestly, I might prefer the 737-MAX10. Before someone yells at me, hear me out. UA puts less first class seats on the A319 and A320 (8 and 12) then they do on the 737 and 738 (12 and 16). The trend is to add 4 seats for every larger variant, as the 739 then has 20 seats. This would mean a A321 would have 16 first class seats while a MAX10 would have 24. Much better chance of an upgrade with 50% more first class seats.
Of course, this logic may not hold up because UA can put however many first class seats it wants on any plane. But I'm just following the trend...