Difference is BIG!
Non-magnetic sumo robots are uses lower powered dc motors and their acceleration can be very noticeable. Where as magnetic, japan style sumo robots uses very high power dc motors and their acceleration is nearly instantaneous.
Also magnetic sumo robots make fast turns, fast stops etc. Magnets help them to stop suddenly. Non-magnetic sumo robots don't have features like like.
But at same time developing japan style magnetic sumo is hard and expensive due to special parts (though you can find lots of parts at our store) So you can think non-magnetic as beginner, magnetic is as advanced robotics category.