Saltwater Fish +) More beautiful, more colorful and a wider variety of fish and invertebrates to choose from
+) Saltwater fish have distinct personalities making them more fun to watch
--) Algae grows at least twice as fast in a saltwater tank
--) Cost much more with the average fish being $12 - $24
--) More Delicate and more susceptible to disease
Freshwater Fish
--) Not as colorful and less of a variety of choices
--) Anyone familiar with aquarium fish will tell you freshwater fish aren't quite as smart making their behavior slightly dull
+) Algae grows 3-4 times slower in a freshwater tank
+) The average cost of fish is $2 - $5
+) Hardier and less susceptible to disease
It's a weighty decision when making the choice between the two types of fish tanks, however it boils down to these decisions... You have to be willing and committed to do the maintenance with a saltwater tank on a monthly basis or your fish will get sick and possible die... As you can see from above, the fish are more expensive with saltwater and the cost of loss will be much greater... Freshwater are hardier, less expensive and you can expect to possibly never lose fish until they pass with old age... If you miss a water change with Freshwater, the fish will be fine however a healthy aquarium will always have regular monthly maintenance...
Personally, I have primarily had Saltwater aquariums and always carefully chosen the hardiest, healthiest fish... I am always available for questions, please post any comments or questions you might have - Mahalo!