J.S.
Ours had a slight back and sides to it, which worked fine for our daughter. Our son was a "husky" boy at that age so he had to sit on a regular potty with the insert for children. My daughter at one point starting preferring the insert over the chair too.
If money is an issue (when is it not these days), I would invest in a good insert for the adult potty. Less mess to clean up :0)
J. S.