As a passenger, I'm hoping for the Max 10. I think that's the best chance for a good F-ratio, and I prefer the Boeing interiors by a slight margin. I'll trade the unnoticeable seat width for other details like the windows, PSUs, and BSI.
I also think this is UA's chance to go all-Boeing with narrowbodies in the long-term. I believe that will be a factor.
They're so close though that either plane will do. Not a big enough difference either way to make a fuss over it.