Some of the moves are harder to do on a smaller step. V steps, horseshoes, skip across the board, pendulum repeaters, etc, etc. You'll get a better workout if you open up the movements and use the larger board.
That being said, I have a small step that works great in my tiny living room. I don't think I could use a bigger step where I am, but if I had the room, I'd want to try the big one.
April