If you're looking for a guy to help RIGHT NOW, then pass on Garza. Jason Hirsh *might* be better in that regard, but he is highly risky too. But if you want an excellent prospect for next year and beyond in a Dynasty league (your first post said dynasty), I'd say go for Garza even though he may not have much value this season. Most other top prospects aren't guaranteed callups and some of them like Gordon won't be called up at all. Rookie pitchers are HUGE risks, so picking up Garza if you are pushing hard to win THIS year might not be a good idea.
I know I'm going to try for Garza in the Dynasty roto league I'm in (with Hirsh as the backup plan), but I'm not going to use either for the remainder of the year unless I absolutely have to.