It's hard to say which is a more efficient use of fissiles. It's easier to burn up all your fuels in the context of an explosion as in Orion than in a saltwater rocket. On the other hand by bulking out the fuel with water to form a larger mass of propellant the saltwater uses it's energy more efficiently at the cost of lower ISP.
But really, if we're willing to leak radioactive byproducts I think an open cycle gas core might be the best bet in terms of not-yet-developed rockets.
http://www.projectrho.com/public_html/rocket/enginelist.php#...