HS players start at a lower ratings point but have more seasons of progression. Growth is limited by age and # of seasons, so the younger a player is when he starts, the more seasons of development. To a degree it mirrors the theoretical growth HS players would have had in college ball. If you go the HS route, it becomes more important to have a few good coaches in the lower minors to maximize development.
Specifically on the higher ceiling, if you draft a HS and College player with an OVR of 55 at time of draft, all other ratings being equal the HS player might have a Projection (ceiling) of 68 and the College guy 64.