Texas2004,
I can't completely answer your question. There are as many variables as there are players.
First there is the individual clubs and their philosophy on the draft. Now days, some are college player ahead of high school player, no matter what. Others, like the Braves, lean to high school players first.
Roy Clark (Braves Scouting Director) told me drafting high school prospects takes more work and pays bigger dividends. He told me "Weren't they all in high school at some point"? It's just a matter of drafting the right ones at the right time.
Projection covers a large area. Obvious is size, body type and projected growth, but everything from mechanics to arm action to blood lines to work ethic plays a role. Sometimes we project backwards. For example it's easy to project a tall, skinny pitcher with good shoulders and long limbs. We have projected based on the opposite. Big fat kids with the above traights have proven to make it big once they got themselves toned up. One such pitcher from Ohio we saw at 275 lbs last winter. He threw mid 80s, but we were very high on him. This October he was 230 in Jupiter, he threw 92 with a very good breaking ball. Reports are he's even higher right now.
Obviously the younger a player is, the more projection necessary.
Normally the easiest way to explain it is:
High School Prospect is higher risk-higher reward.
College Star is less risk
If you look at last years MLB All Star rosters, you will see the great majority of those players, did not play college baseball.
You got the Josh Beckett's and you got the Mark Prior's. The Kerry Wood's, the Roger Clemen's. The Pedro's and Johnson's. The Maddux and the Pettite.
All things being equal the younger player is the most projectable and the best pick. IMO, many will disagree.
Then again does he project to become as good as Mark Prior?