I think it is a different feel and it depends on what they like it "to be". If they want a skateboard feeling, then 3 wheels and a ball is more like it, but if they want a classic scooter balancing forward and steering, then two wheels is the thing. I have bought my daughter the 3 wheel one for her 4th birthday and I will buy her the ball as an "extra" for Christmas or next year, when she will be proficient and in need of a new challenge
Good luck! I hope your choice is very successful!