Ay, tough question. I think Gallardo will have a much bigger impact and will have much more success when he breaks into the bigs, but the ETA is in question. He's going to finish the season at AA, and when the AA season ends it is not that unlikely he'll get a call up and pitch out of the bullpen a little, depending on where the Brewers stand at that time.
Petit should get called up in September and be with the Marlins to start 2007, my guess in a relief role. With Dontrelle, Olsen, Johnson, Sanchez, and Nolasco all earning starting roles, I can't imagine Petit as much more than a MR for his first few years. I don't think he has good enough stuff to make guys miss and think that at best he will be a back-of-the-rotation starter.
Gallardo has been the most dominant pitcher in the minors this season, and has mastered A+ and AA at 20. He was a good pitching prospect prior to this season, but control and a third pitch after his plus-fastball and plus-curve were in question. He more than answered that this season, showing great control at A+ (2.6 BB/9) and still much improved control at AA (3.1 BB/9), which is inflated his his 4.2 IP, 5 BB performance a few weeks ago. Anyways, he has come into 2006 with TWO pitches to add to his repertoire: a change and a slider. Both are currently ML average, and most don't expect his change to get much better. His slider is what they are talking about the most, as it has loads of potential and could end up being his best pitch. SPs with three plus-pitches are hard to find.
Gallardo's call-up will boil down to three things: ability to master all his pitches, especially that slider, experience, and performance. Barring a knockout September and an even better ST in '07, Gallardo will start next season at AAA. But don't be surprised to see a midseason call-up. I think Gallardo could settle very well into a #2 spot, and, at worst, a #3.
So to sum up all this crap, I'd definitely take Gallardo over Petit.