Freshwater IMO for many reasons.
1) Freshwater is cheaper
2) More diversity (most saltwater fish are reef fish, freshwater fish come from lakes, streams, rivers, ditches, coldwater, warm water, caves etc.) Keep in mind that around 42% of all the worlds known fish are freshwater, that is a lot considering only one percent of all water on the planet is freshwater.
3) Most saltwater fish are wild caught, sometimes through very destructive means.
4) Saltwater is more difficult to maintain
5) The color of the fish. This one may seem odd but it is true. Freshwater fish can pretty much change color, healthy fish display wonderful color and it changes with their mood. You can often tell if a freshwater fishkeeper is doing good or not based on color. Saltwater fish on the other hand always display the same color no matter what.
With all that said, I would like to try a salt tank some day but there are soooo many freshwater fish that come higher on my list than any saltwater.