I applaud your attempt to find a unique manner in providing carbonated beverages to your clients. I do prefer fountain drinks over bottles or cans when I do drink a soda.
But here is another thing to consider. If you are a legal operation, the Sodastream is not. It does not meet NSF guidelines in construction etc. Of course your health department may never comment on it, but I assure you that if mine came to my commissary and saw it on the shelf they would probably say something.
Just another thing to consider, other than just pricing.