I currently have them both. Got a GSM Gnex from google play store and Pre ordered Evo LTE. Evo blows away the Gnex in so many areas. Better and brighter screen, better camera quality and stabilization, better batery life, better build quality, better call quality, smoother UI experience... Animations are VERY smooth (Pre Jellybean). Expandable storage, much louder speaker, sound and notifications. I sadly have to part with my Evo because of Two reasons. The main one being sprint data speeds in MY AREA. Painfully slow...to the point that I would wonder why I paid sprint for the service that I was getting. Sprints 3g speeds make the phone useless in the areas that I am in during the day. The other reason I chose the Gnex was stock ICS, no carrier delays in updates and better feel with the UI and with the phone in hand. The phone seems to disappear when watching videos. The phone and the UI feel as if they are one. You have to use the phone to understand.
Hope I helped.