What's the difference between these two rankings? Also which recruiting sites (scouts or rivals or someone else) seems more creditable when predicting the future impact players?  

These rankings seem more based on the offers kids get.  For example if a one kid is getting offers from the likes of Texas, USC, LSU etc. the get some stars.  If a similar player is getting an offer from commonwealth of maryland state technical college he's likely to be overlooked. Just look at Texas' recent recruits, before a few weeks ago only a couple were ranked, now all of a sudden twice as many are "bluechips"?  Now this doesn't say they're wrong, just that more of the actual scouting is done by the Universities.  It's a reasonable way to pick out players, but its all correlation.  Rivals knows the elite schools usually recruit the elite players.  Stats play a role, but I think they are sort of secondary when it comes to rankings.  Thats why you'll see Sheppard as a 5 star QB when he threw for less than like 800(?) yards.  They give him the benefit of the doubt because of the schools that are after him.

As far as who I prefer...I like rivals, but not for a particular reason.