Startup/maintenance costs tend to go like this, cheapest to most expensive, using a standard 55:
FW, non planted.
Marine, fish only
FW planted
Marine with live rock
Reef.
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.
I have both.