N. erythrogaster would be my second choice to clarkii. They are beautiful and stay relatively small. I have found them to be a little bit more finicky captives than N. sipedon and N. fasciata, but once you get them going they do great. They are definitely more docile as WC adults which can be a plus. There are 6 subspecies, 4 of which are in the U.S.
N. sipedon and N. fasciata are great pets if you can get them as babies. You just run the risk of them growing into very unattractive adults unless you know what the parents look like. I have seen some N. s. pleuralis that are stunning and the lighter N. fasciata confluens are gorgeous.
All the other Nerodia get bigger than 3 feet or are protected in the wild.