It is a well known fact that the draft is by no means an exact science. Someone like Albert Pujols is drafted in the 13th round, while people like Matt Bush are drafted first overall. Now even though you can never be sure of what you're going to get, you can use information to get a pretty accurate guess.
That being said, with a late to mid 20's pick in the draft that the Giants have, I think it would be best for the organization to draft a high upside, high risk high school prospect, rather than a more pollished, low upside college product. Look at the farm system now, Joe Panik, our first round pick in 2011 out of college, is about to begin AA, while our supplemental first round pick, Kyle Crick, is only one level behind him, has much more upside, and could very likely end up being a front of the rotation starter. Now, the obvious reason that Panik was drafted earlier is that people felt he had a better chance to make it to the big leagues.
I recently looked at the drafts from 2000-2008 and looked at the 20th overall pick through the end of the supplemental round, and then compiled two lists; one for high school prospects who became notable MLB players, and college prospects who became successful MLB players:
High school 20th overall-supplemental first round:
Adam Wainwright, Kelly Johnson, Noah Lowry, David Wright, Denard Span, Jeff Francoeur, Matt Cain,
Chad Billingsley, Jarrod Saltalamachia, Adam Jones, Phil Hughes, Colby Rasmus, Rick Porcello, Ben Revere, Travis D'Arnaud(not a big league player yet, but has huge potential.)
College 20th overall-supplemental first round:
Bobby Crosby, Jeremy Guthrie, Joe Blanton, Mark Teahen, Chad Cordero, Carlos Quentin, Jacoby Ellsbury, Matt Garza, joey Devine, Clay Buchholz, Jed Lowrie, Ian Kennedy, Joba chamberlain, Chris Perez, todd Frazier, JP Arencibia.
The first list has less people but more talent. The First list has more of the front of the rotation starters, and more of the all star type players. And lets not forget that THE Mike Trout was drafted in the late first round of his draft.
In the end, it comes down to what the Giants need, and right now, the Giants need more high ceiling guys, and less safe picks.