Rockwall would probably win out, but it depends on your reasons. Forney is more country-fied. There isn't a whole lot going on out there. Yes, there are some shops and things to do, but Rockwall has a lot more to offer the family.
As far as schools go, Rockwall ISD is TEA-rated as Recognized District
Forney ISD is rated Exemplary District by TEA. So, you can live in the country and get an outstanding education.
So, depending on what motivates you, you can answer either way.