Damn that is a hard one. Last week, Vick Ballard probably got 2/3 of the carries on the Colts. I've never been very high on Donald Brown (guess the Colts have never been either) so as long as Fred Jackson is healthy and playing, I'd rather play him. He has a higher chance for a homerun play than Brown.
Honestly though, it's a coin flip. Don't expect much from either player so don't agonize over it too much.