I can sympathize with you cause I'm a Sproles owner also, only thing is I don't have a viable 2nd guy cause I'm in a 14-Team league.
Anyway Steven Jackson's on his way out in STL and Decker vs Bowe is pretty comparable. Bowe for S-Jax is just not a good deal for you because Jackson's ceiling is like 15 pts. At least with Bradshaw you know he has the capability of putting up big numbers, albeit inconsistent.
A lot has to do with whether you started Bowe last night (which I'm assuming you didn't) because then you only have Harvin, Julio, and Cobb that must be your starters (if you deal Decker). I think you have the depth to do it and those 3 wouldn't be too shabby of a starting lineup this week.
I know Bowe is hard to rely on cause the KC offense is horrendous but they are most likely gonna trail in every game, he's gonna get targets and on top of that he's still the #1 and he's young.
Because of that the difference between the players is like this:
You get the point, I would do Decker for Bradshaw.