Our association seems to prefer having their new guys work at umpire and I believe this is a good approach because it keeps new guys in the middle of things and it requires them to know penalty enforcements. Most of my games were at U though I did a handful at HL. They will put guys with a few years of experience in at R for youth games and veterans usually work at BJ for Varsity.
I really enjoy working at U and being right in the middle of things on every play. It feels like home to me as I was a center and linebacker in high school. I even had half of a tackle on a running back last year.
Working the wing is not bad either...at least when the volume on the coaches is turned down low.