If you're looking for a little extra cash, the Type II gas giants used to, and AFAIK still do, pay out quite well. I've seen it mentioned somewhere that Frontier have plans for gas giants in the future, so maybe that's the reason for the much higher pay out for the two's.
On the other hand, if you're looking for something a little out of the ordinary, try looking for Helium-rich gas giants. They're on the rare-ish side, but if you come across one system that has one or more, then the chances are good that one or more of the neighboring systems also have them. This type of gas giant only forms in young star forming regions where the age of the stars are ~1 billions years old or less. One of the reasons I like coming across these is that the systems they're in are usually crazy large with dozens and dozens of bodies in all kinds of strange combinations and orbits.
If you're out looking for the real rare birds, try finding a Water Giant. These are very rare to come across, at least for most us

. This type AFAIK is just about one of the rarest bodies to find in the game.
Happy hunting
Reido, (CMDR Andrew Reid)