The seat and service will be basically the same.

However, I would slightly prefer the 787 because of the higher cabin pressure. The 757 is blocked for about 10 extra minutes flight time, if that is significant to you.

edit add: you can spend some or a lot of time looking through the following threads as well to get more ideas

Consolidated Where to Sit on a United Plane Threads

Everything You Want to Know About Where to Sit on a United Dreamliner 787-8

https://www.flyertalk.com/forum/unit...200-16-bf.html