I voted Tanner. If he has the full grasp of the offense and is playing well, I don't think you can bench him for a year and expect him to wait. Of course, that assumes he is healthy and is looking like a John Beck, Max Hall, or even better.
You could put all your eggs in the Taysom Hill basket, but then he could be injured the first game next season, which means you deprived Mangum of meaningful snaps in the offseason for no reason. Of course, that is always the case when the starter goes down, but at this point it seems obvious that a running QB like Taysom is simply more likely to have a season ending injury than a drop back QB like Tanner.
I love Taysom, but the only reason I think coaches should or would play him next year would be if Tanner simply doesn't pan out--which would be a different set of facts than the hypothetical gave of Tanner playing well.