Since you say the rest of your team is a no-brainer... would it be fair to say that this flex position is a spot you've struggled to fill all season, and may have had a few duds here and there? You've survived it this far and are still playing in the playoffs, right? If so, then I'd say roll with Webb because of the potential upside. He's not the safe pick, and if this flex is a position that you need at least 10 points out of, then I'd look elsewhere to Choice or James (I'd probably go Choice cause the Skins will be a joke this week). It's hard telling without seeing the rest of your team, but if you can afford getting a goose-egg from him and still field a somewhat respectable team, than I would go for it.
I remember Colston's rookie year when he was eligible as a TE... dude killed it for many owners that year. He could prove to be huge going out if he does well. It was well said above that it would make for a great story.
12 team, 0.5 PPR, 10yds rush/rec, 25 pass yd, 6 point TDs
1QB, 2RB, 2WR, 1TE, 1WR/RB/TE, 1K, 1DEF/ST
QB: Vick, Freeman
WR: Marshall, Gaffney, Amendola, N.Washington, VJax
RB: Peterson, Foster, Charles, Portis