That's a question that I've been pondering as well. Both were available in my league and I opted to go with Roberts based mostly on the fact that he's proven. Stealing bases isn't all in the speed... it's also about reading pitchers, and Roberts has proven that he can do it consistently. Plus Roberts has 7 games this week, which is nice.
However, I can't really see Tavares losing much time in the OF after he gunned down 3 players at the plate in the weekend series against the Cubs. He didn't have a great week at the plate, but if he stays in that lead-off spot after Berkman gets back, he could have near-Juan Pierre value.