Benson put up his best year last year. Ronnie Brown and Cadillac Williams both had their second best years. You'll notice that all of these backs handled significant duties in college. Drafting Benson, for example, gives you a guy who's got 1112 carries in his college career. That's about 4 years worth of work in the NFL, so he's not exactly going to be a rookie. This is especially important considering that running back is the most physically demanding position in football. Williams also has significant mileage, but Brown has the least.
The most comparable to LJ is Brown - similar size, similar speed, and similar capabilities. By the way, it's ridiculous to blame Johnson for the coaching decisions that surrounded him in his career. Paterno doesn't like to play underclassmen if there are upperclassmen available. Johnson was always a standout player - even as an underclassmen, he excelled whenever he had the chance to play. He had limited time at running back and special teams but he was always good. I personally think he's the best back on KC's roster, but Vermeil doesn't see it that way. Johnson did what he could to prove himself this year - just like McGahee. McGahee who, by the way, only had one year in college. McGahee, by the way, who is ALSO better than all three of the backs in this draft. Even with their NFL work, they're still more of a rookie than Cedric Benson.
Don't even act like a player's sophomore and junior years come into play all that much. It does happen - like with Mike Williams - but it's the exception, not the rule. What did Alex Smith and Aaron Rodgers do last year? They both had good seasons, but you don't even hear it being mentioned. If stats were all that mattered, then Timmy Chang would be a sure first rounder.