we used a B22 omega enlarger for many years in a gang darkroom, and found it to be more than satisfactory. In fact it was very popular and several people bought one of the same type when setting up their own darkrooms.
I have never heard that enlarger be refered to as a "student enlarger". The D series, is much larger and a very good stable enlarger as well.
If you want to move up in format size then the D would be an advantage, but if you are getting the results you want from the B22d, why change.