Your best bet for information is Rossi & Rossi's Snakes of the United States and Canada: Natural History and Care in Captivity. As I recall, they rank the smooth green snake at 4 and the rough green snake at 5 on a scale of 1-5, with 5 the most difficult end.
Smooth green snakes have turned up in my back yard, but I only tried to keep one. That did not turn out well. I used to know a guy who kept smooth greens successfully on crickets, though. My take on the smooth green snake is beautiful coloration but small, nervous, secretive, does not like to be handled. And the insect diet is another minus point. So I do not intend to try keeping any again.
I caught a rough green snake in Louisiana but did not keep it.