Sander = smoothing/polishing
Grinder = shaping/stock removal
Sanders are good for deburring,cleaning edges off that sort of thing.If you want to remove stock to a layout line or remove weld or cutting slag then you need a belt grinder.
A belt grinder will have more raw horsepower and run much more aggressive belts.The one we had at work runs 3hp and carries a Zirconia belt.With a 36 grit belt it peels little fish hook shaped "dust"off work pieces thick enough for them to form.
If given the choice between the two I would take the grinder.Simply because with a belt change it will do everything the sander will too.
I just need one more tool,just one!