I use 120 grit. When I first got into stationary sanding machines I thought the finer the better.. Fine belts burn and don't last .. Also, 120 on an edge sander or a Wide belt, leaves a better finish than when sanded with a hand sander.
No idea if that is true..lol .. But its the way it seems to me ..
Edge sanders are awesome tools. Mine only has one speed, but I still love it Use it more than I would ever have imagined.