Ellefson clearly has better technique and can play with more precision; Cliff is very sloppy. That said, Cliff is a pivotal musician (not only bass player) in thrash and metal history, and his contribution is unmeasurable. Cliff's attitude and tone are more important than his sloppiness, and actually that sloppiness in part is what make him, and metal of that era, so special... It's not the perfected ProTool metal (and music in general) we hear today, but humans sounding human. And all that said, Ellefson is amazing in his own right, and also a big contributor to metal bass.