I think lead loading is more of a traditional method of achieving the same ends, but filler has a bad reputation from mis-use. I think (but I don't know) that lead-loading might well be better from the point of view that the bond with the underlying metal is better so it's less likely that moisture would get underneath it and rot as it can with plastic filler.

One advantage I can see for plastic filler is that if you're trying to use it on a place like the bottom of the wheelarches, if a big lump of it drops off and lands on the back of your hand, it won't hurt quite so much as molten lead probably will. (Not that I'm suggesting you'd use big lumps of course).