I would agree that going with a hand-finish or a flap sander would be better than a belt. If the belt is wide and the plate is not very flat, you may end up with a heavy polish on the high spots and have diffculty getting a uniform finish without removing a lot of surface. I would expect the Scotchbrite belts to be much better than grit in this regard.

My vote would be for leaving the welds exposed and not blending. I did this with a small bathroom towel rack and thought it looked really good as-welded.