Hey Janna, As far as I know I don't think there really is any difference in stability balls. They are all round, all made of a tough plastic that holds up to 300 lbs, and aparently they all can be popped by two and three year olds!
I would really just suggest getting a cheaper one, it works the same. I believe that KMart has one for around $15. At least they used to. Or try megafitness.com. They have one for $16.95 right now, but it's for short people, like myself. Or they have a larger one that's $17.95. I have no idea how tall you are but they show teh scale so that will help. Plus there is no shipping charge, so that is very nice too.
I hope this helps!
Kathy