I have both glass and a wet mount system, I'm sure I will use the regualr 120, glass 120, and wet mount film in the future. The glass holder (at least mine) has diffused glass on the light side, which gives the same effect as a diffuse enlarger head will give you, it is also not anti-newton so you may get newton rings. Even on relatively flat film the regular 20 holder is a weak link. Good for rapid mounting to cull the good from the bad, but that is about it. Wet mount gives flat film and no newton rings, and no diffuse glass so the edges are sharper like with the regular holder.
Also note that a new glass holder costs as much or slightly more than my wet mount kit. Even on ebay they go for around $200+.