I'm more of a lurker here than anything else, but since I have these two on my fantasy team and I follow them closely, I'll put in my .02
I think Evans definitely has the bigger upside here. If you think about it, Evans gets to the hoop at will and unless the defense is setup to specifically stop his penetration game, then he'll have his 20/5/5 night. Once Evans develops his midrange and outside jump shot, he'll become an unstoppable force. On top of that, he's already a top notch defender and he'll only get better as he gets older. Remember, he's only 20 years old. Kobe or LeBron didn't have a jumpshot when they entered the league and look at how near automatic their shots are now.....scary...
Jennings on the other hand will still be a very good player in the league, but I foresee his success amounting to that of Mike Bibby. He'll be a legitimate scorer and play maker, but not a great scorer and play maker. He's already spent a few pro seasons in Europe so he's had plenty of time to grow while playing above the college level. He's fast and somewhat flashy, but if you put a good sized point guard on him that can defend well, he's pretty much canned for the night. He's a smaller PG so bigger guards get around him a lot easier than say a Tyreke Evans. One other thing is that he shoots A LOT and thus far has brought my overall shooting % down for my fantasy team, I've been wanting to trade him for someone a little more consistent for a few weeks now.
One last thing...(which may or may not be worth mentioning)
Jennings seems to lack the professionalism and dedication that Tyreke has. Remember the video conversation that was uploaded where Jennings was saying this and that about the Knicks and Duhon and blah blah blah? Now I'm not saying that other players, including Reke, don't talk like that, but just that in itself shows a lack of respect for the game.
Tyreke on the other hand seems way more dedicated. He's basically a basketball drone forged into the life of basketball by his older brothers, and focuses on nothing but getting better and better. To be that young and show that type of poise, professionalism, and mentality to keep on getting better is already enough for me to believe he'll be better than Jennings. Hell, if Donte Greene had half the maturity and dedication to the game that Evans has he could very well have a starting position on lockdown.
Given their comparative ages, what they've been able to do in the league, their overall experience thus far, and their command/respect for the game I think Tyreke wins in the "higher ceiling" challenge.