Salt water is a lot prettier and prolly doesn't cost that much more than fresh water.
Sam's got a nice salt water tank at his house, so I'll defer to his opinion and knowledge of the difference.
I've wanted a salt water tank forever. But I want one with ~local~ stuff in it, not tropical. Apparently this is a nontrivial problem. The water has to be chilled, not heated, and the chemistry is complicated.
But it would be a gas to be able to pick up critters off the beach and have a tank in the house for the collection.