The whole debate in vertical/horizontal hooks is sort of gray area, as khov says. I tend to throw my low punches vertical as it feels more natural and the surface area really doesn't matter when you're hitting a target thats bigger than your hand no matter where you hit (AKA the surface area is the same either way). I never throw hooks to the head vertical though, but I can see how it's possible to use it in an advantageous way when thrown to the temple. The temple, being a high target, is easy to mis judge height wise. Throwing vertical gives you m ore room for error on the punch and it will still connect correctly. Using it to the jaw though means you're trying to hit more of a lateral target, so throwing it horizontal (palm down) give you more room for error. Also, as far as MMA goes, throwing them vertical makes it easier to clinch on a missed shot (like Fedor does all the time). You can throw them both with the EXACT same technique as well, the wrist is almost a completely independant joint of the elbow and if it's done correctly can it can be done with very little to no rotation on the forearm and elbow (you can throw both of them flat very easily).
EDIT: Khov, whats the first? Jab?