What does "intention" mean?
Cetana is executed by action, speech and thinking. 


Samma Sankappa - Right Thinking 
Samma Vaca - Right Speech 
Samma Kammantha - Right Action 


So the difference is Cetana includes speech and action where as Sankappa by itself only includes thinking. 

Please note that "Right" here is explained properly by Samma Ditti and doesn't mean right action in general sense (eg. Teaching, Nursing).