I find that I'm fascinated by SW
tanks. So many different types of critters can be found living in and around the rocks and sand beds. When you toss in a couple of fish, it can be really exciting to see. I recall seeing a pic of a cleaner shrimp doing his cleaning thing on a large fish.
That being said, I have an equal fascination with planted FW
tanks. The fact that you can keep aquatic plants in a tank is really cool.
So, for me, though I keep a FW
planted tank, I like SW
tanks just as much.