You cannot get the Enterprise until you have at least 15 users.

We have the Small Business and cannot get the Enterprise until we get to 15 users.

The only difference that I know of, is client licenses are $250 for Small Business and more like $80 for Enterprise.

There is nothing really to decide if you are <15 users you can only get the Small Business.

If you are >15 then you can only get the Enterprise.