I'd go Santana, but I couldn't argue against a Peavy backer in a 5x5 roto league. I'm a little biased to boot, and the rooting interest is more than worth a point or two in #1 SP ranking. I doubt I'd take either with the top 5 picks overall.
If the league counts complete games though, all things being neutral I'd say Halladay would be the #1 SP. My points league counts them, and he looks like the most dominant thing out there with that category.
I'm lucky I have Halladay and Johan, but Halladay is the real stud SP in alot of leagues.
* Steroid induced record