United's 757-300s have 30-31" pitch while the 737-800 only has 30". Not that it makes much of a difference, but worth noting. The 753 also has power outlets in all rows while the 738 only has them in rows 1-21 (except NEXT planes). In addition the 753 is a rare plane and such a novelty to fly on. Be aware that the 753 does not have seatback screens/Directv anymore, you have to use PDE. In addition if you are in the back of the plane it may take a while to deboard.

Edit: If you are traveling in F, you'll get the newer seats introduced in 2015. On the 738 you will have the old CO seats however, unless its a NEXT plane.

I think you made the right choice here. I'd take a 757 over a 737 any day of the week.