I'd take Terrence Ross instead, but there should be some big guys at that first pick that will make it tough to draft small.
As far as Ross vs Rivers and upside, I'm gonna go with the kid who wasn't raised by an NBA player and coach, with an indoor regulation court (or whatever), practicing in NBA arenas with nba players all his life, with 2 D-1 older brothers to play every day with.
Rivers has some natural, physical talent, like a quick first step and longish arms for an undersized 2. But much of what Rivers brings is a refined skill set developed under ideal conditions. I think he is closer to his ceiling than most freshmen, and he also should be a sophomore, age-wise.
I also think Rondo + Rivers = 1 too many diva cooks in the kitchen.