If the choice is between spending the same amount of miles for Saver F or Anytime Y (or whatever it's called on United), I would choose F.

And, I have a 23 month old. The extra leg room is great because he can't reach the seat in front to kick. If possible, position one parent in front of the child and one next to the child. Much better than having one adult across the aisle.

(If the choice was between Saver F and Saver Y, I'd choose Y.)