College vs. High School
The easy answer is to say case-by-case basis, right? There are collegians and high schoolers at every position, at every level, and from every background who make it big in the Majors, who get themselves a cup of coffee, or who bomb miserably, and it's the same with international signees.
Let's stay away from the international guys (specifically the players who are not eligible for the June draft) for now and focus on the pluses and minuses of collegians and high schoolers. Keep in mind I'm just rattling them off the top of my head -- I'm sure you can come up with a number of other good ones.
First, the college guys.
+ More polished.
+ More experienced.
+ More mature. (not always the case)
+ They've had better coaching.
+ They've played against better competition.
+ They'll (ideally) get to the Majors quicker.
- Their ceiling is evident. After all, it's not like they're going to get much quicker or stronger, or start throwing the ball too much harder (unless something is changed mechanically).
- They may be uncoachable in the minors because they think they already know it all. (I've most definitely witnessed this with some unnamed Jays farmhands.)
The high schoolers, now.
+ High potential/ceiling. They could very well still be growing, adding more mph to their fastball, learning new pitches, and learning to be excellent defensively.
+ Youth is on their side. Four years of seasoning in the minors for a high schooler means a 22-year-old Major League rising star. Four years of seasoning in the minors for a collegian means a 26-year-old who better start doing something quick in the Majors or else he's gone.
- High potential for injuries from 17-year-olds throwing 12-6 curves and 94 mph heat.
- Immaturity on and off the field -- they're away from home for the first time, and they've just received an enormous signing bonus. That spells trouble.
Me, I'm a pro-college guy with a few high schoolers sprinkled in. I'm not a patient fan, I want to know within 3-4 years or so if a guy's going to make it or not.
Let me add, though, that the Lugnuts pitcher who's the most impressive right now, high above all of the college guys in mound savvy and stuff (let alone potential), is 19-year-old Drew Hutchison, a 2008 15th round pick from a Florida high school.