I have a similar choice with Jennings vs. Garcon for my flex. I have decided that the choice is a risk/reward play. I think Garcon is a lot safer bet to get 10-12 points, but Jennings is riskier bet with possibly higher reward. For example, I think Garcon is 70% likely to get 11 points with a range of plus or minus 3. I'm only 50% confident that Jennings could get about 10-12, but he could also rush for 175 yards and 2 TD's, OR MJD could rip off a 60 yard run on a 3rd down and then Jennings only gets 3 more carries all game.
My game is a close match up, where I think every point could count, so I think I'm going to go with the more sure thing in Garcon, even though he has a far lower chance to have a big day. If your game is similar I would recommend the same. If on the other hand, you are a big underdog, I'd go with Jennings who has a small but real chance of going off for a big game.