My experience, from having done both sides:
With 20M College I see a full 500 prospects. I usually manually rank a top 50 or so. When I draft I sign almost all my 25 players which lets me stock my Rookie league team and move my top draftees to whichever higher level suits them. I usually end up with minor-league capable players down to around my 15th pick. The rest are usually cannon fodder pitchers.
I had another team with which I tried 20M High School. Since they're all 18 year olds, the total number of projected prospects I'd see was only around 200. All the rest had "?" projections. I would end up manually ranking about 25 or 30 and the rest was pointless. By round 8 to 10 I'd be drafting players who weren't even good enough to ever get out of High A. My minor league teams ended up being complete trash. I started moving many of those project players down to the bottom of the rankings and drafting "?" guys just to see if I'd get lucky with one of those unseen low-round lottery tickets.
So basically, if all you're looking for is to score with your top two or maybe three draftees, go High School. Otherwise go College.