You've got to go with Carpenter here. Granted there is always the injury concern, but if he makes 30-35 starts you can lock up a top 5 finish in the Cy Young balloting. While neither is technically a sure thing and the difference in draft picks is negligible, I'd much rather have Carp.
I have what may be an unhealthy man crush on Ubaldo Jimenez, but that said, I'd take Carpenter. He is currently the better pitcher of the two, and he'll cost a round later selection (which is really just gravy at this point). As much as I'd like to say Ubaldo, I can't, go Carp.
I'll dissent and go with Ubaldo by a hair. They're very close and Carpenter may have good upside, but he is too much of an injury risk. Jimenez has the body type that he'll have a minuscule chance of getting injured. Plus I like the Ks better.