That's a tough one.
* For starters, keep in mind that Felix Hernandez is a year and a half younger than Matt Cain.
* Hernandez ranked as high as Baseball America's No. 2 prospect entering 2005 and at the age of 18. Cain ranked as high as No. 10 when he was 20 for BA. Although these rankings are a little fuzzy to compare because each class of top prospects is different, I think it's fair to say that Hernandez has always been thought to have the heigher ceiling.
* They are both VERY (as in Cy Young potential) good. I think Hernandez will take it to the next level; Cain will keep it up.
I think Cain will be a little better in 2007, but after that, all bets are off. Hernandez still has the higher ceiling in my mind. There's no clear choice, you should just go with whomever you think fits best on your team or what your gut is telling you. Just be comfortable knowing that you can't really make a bad choice.
Mike Pelfrey > Matt Garza