Kolb should post solid save numbers, but he has downsides and risks:
1. Not a high k guy, so he'll only really help you in saves
2. Maybe Smoltz can't handle starting, so he goes back to closing.
Zambrano is a high k guy who should, at a minimum, help you in WHIP, K, and ERA, and I can't imagine him hurting you in wins. Even if he doesnt' become a stud, he's a very solid starter.
If you think you can't rely on the wire to cover saves, than I guess you want to keep Kolb, but I'd say you're probably below average in all pitching categories right now, so improving in at least three, and possibly four, is nice.
One more thing. There is no way you're going to win saves anyway, right? Someone out there must have a couple of stud closers. Take a look at the other teams and see what place you think you'd end up. If losing Kolb drops you from 4 to 6, but gaining Zambrano can up you to third or fourth from seventh in the four others, it would be a no brainer.