Like most projects, whatever tool works the best at the time. I like the 4" cutoff wheels on a hand held grinder. If I'm seperating a spot welded seam, drill out the spot weld on the piece your removing and then air chisel it off. If the layer your removing is really rusted, I don;t even bother cutting at the spot welds, the air chisel will pound it right off. I've even used a cutting torch to burn off the top layer of a seam.
Just make sure you protect the glass if you use the grinder to cut, the sparks will imbed in the glass and you'll have a mess.