The Blair cutter is the hot ticket. I've used the cheapie cutters and had decent results but if you need to do more than a couple spots the things don't last so you're better off spending a bit more and saving yourself the frustration. Also get yourself a set of the panel splitters - these are hard flat blades you hammer between the panels after drilling out the spotweld to split the panels (you'll rarely drill out the weld perfectly!!)

I found the cutters break if they are able to wander. If you drill a dimple or even a hole right through the panels so the pilot stays put the cutter will work like a charm. Also keep the drill speed at a medium speed - slow enough not to heat up the cutter but fast enough that it doesn't jam in the material.