Definitely Ebron... Out of those 2... I've already read a few articles on how Gates' understudy, Ladarius Green, will see increased snaps this season. He is about 10 years younger and several steps faster. After studying the game under Gates for 2 seasons I think he is ready to make the leap.
If anything you should consider Green & Ebron instead... Both are worthy candidates. I have Green as my TE sleeper pick this year. Rivers loves his TE's, so I imagine if Green shows he can handle the role then he will be a solid fantasy starter.