John Sickels devotes his life to prospects and how they progress. He also will look back on how a prospect progressed and how they are doing now in the majors.
I won't copy the whole piece, but here are the comparisons he came up with that Ethier has through his age 27 season:
SIM SCORES: Richie Zisk, Bobby Higginson, Sam Chapman, Aubrey Huff, Jacque Jones, Bill Nicholson, Jim Edmonds, Jason Giambi, Larry Doby, and Reggie Sanders.
PECOTA Comps: Kevin McReynolds, Tony Oliva, Bobby Higginson, Gus Bell, Matt Holliday, Wally Moon, Reggie Smith, Al Kaline, Magglio Ordonez, and Bruce Campbell.
Well worth a quick read.