Judging a baseball player is an extremely hard task. This is the reason for why baseball teams call getting two out of fifty draft picks to the major leagues a good draft. So what do you look for in a ballplayer? Everyone has a different opinion. For example Billy Beane looks for college hitters with high OB and SLG percentages. John Schuerholz(that's how they spelled it on the Braves website) likes drafting young and big projectable arms that happen to be from the south, this is how he produced Smoltz, Glavine and others. Some Gm's have more of a tendency to take college players and some GM's like more projectable high school players.
The truth is currently there is no correct method, both are pretty much a tossup because even if your team takes only high school players, out of all 50 players you drafted there's a huge chance that one will make it to the MLB if only because draftees come in bulk. What would I look for in a player? I would look for a few things. A mixture of scouting and sabermetrics. Basically you don't want to draft a player with bad mechanics because it's very hard to improve with bad mechanics and it could make a player more injury prone. You actually want to draft players with perfect mechanics and therefore it'll be much easier for them to reach a high level of play. Just look at Albert Pujols, he didn't go in the first round because he could only hit and he had a good, but not great eye. Instead the guy with the perfect swinging mechanics and the perfect baseball mentality dropped all the way to the 9th round, where he was a steal and is now proving the scouts and statisticians wrong. The scouts because they wanted to find the next Alex Rodriguez and statisticians because they failed to look beyond the box scores on this one.
By scouting, not to judge talent but to evaluate mechanics is the right way to scout because you aways need someone's input on a player they have seen and not only judge by thea guy's numbers. Another thing is that you have to make sure the player is mentally able to play in the major leagues, i.e. Lenny Dykstra, and not turn out like Billy Beane who didn't know how to fail and that ruined his career. Therefore I would test player's mental stability, test them physically, i.e. examine eyesight, strength, bat speed and reflexes along with statistical studies and scouting assessments. Wouldn't you want a player that bats the exact same way as Manny Ramirez o Ken Griffey Jr.? With the idea that I'm proposing, scouts main task would be to gauge a players swinging/running/pitching/fielding mechanics because you will then be able to find the player with, lets say Mark McGuire's swing, and then of course you'd draft him. By weighing all sides: scouting, stats and testing you'll be able to find the best players available and therefore build a terrific core of players for the future, which is now a necessity given baseball's current state.
Therefore it doesn't matter whether you draft high school or college players as long as you get good results and have a system that works. Like this chart, that explains what happens when you have a system that works.(I took it from the internet)
As long as you have a solid system that works you'll succeed more than not(in this case hopefully 5-10 per draft instead of 2). This is needed in baseball since the draft is such a crapshoot nowadays.