Well Freshwater is the easiest and cheapest to keep (it's rather ridiculous to claim otherwise), but I would personally say that saltwater is more fun to keep in terms of color, diversity and just uniqueness. There is also a huge range of variance in difficulty in a saltwater tank. A 20 Long FOWLR with 3-4 fish isn't much more maintenance than say a 20-30 gallon well stocked Freshwater tank. A 125 gallon plus fully active reef loaded with corals, tangs, angelfish, etc. on the other hand is significantly more work and expense.
Startup/maintenance costs tend to go like this, cheapest to most expensive, using a standard 55:
FW, non planted.
Marine, fish only
Marine with live rock
There are varying levels of tech that can be added to each to raise the cost, of course, and cost will vary with the size of the tank. FW planted and Marine with live rock may be swapped.
In terms of interest...well...depends on what you really like seeing. Getting to see animals reproduce is more difficult in marine setups, though possible. Plants in marine tend to be functional rather than pretty, but corals are prettier than most flowers. If you like seeing hitchhikers and microfauna, a reef with lots of live rock is great. If you want a glass box that you can control the contents, freshwater will be better.