I use 0 advanced, but 20 IFA. I find that I very rarely trade FOR prospects (I recently made a big trade for a prospect, but only because I remember going after him in IFA but not having enough to get him, so I knew he was a beast), but I often trade them away. One good thing about having 0 advanced is that I find myself not trading for those mid-tier prospects that, IMO, other owners often overvalue. Like MikeT, I would rather just spend 2M in FA to grab that 4th outfielder than 15M in advanced to try to figure out if a prospect is going to develop into a 4th outfielder.
I think you can usually figure out when a prospect is going to be a star. If he's 18 years old and his splits are in the 60s, he's usually a pretty good bet. Whether his control ends up to be 82 or 86 is kind of irrelevant in my book, because you want him either way.