Beck has more upside because of his ability to throw on the run (and mobility), which is key in Shanahan's offense. With that being said, I think they made the right decision going with Rex. Beck needs more time to grasp the playbook, while Rex is very comfortable with it.
Rex is on a short leash. Expect Beck's name to be called after a several bone head Rex INTs.
"**** it, I'm going deep!"
-Rex