Brotulas tend to be very cryptic, and once added to a reef system my never be seen again. They are neat fish with some very interesting behaviors, but being able to view them can be difficult.
Flounders are some of my favorite fish. They canbe quite colorful and have the ability to change color very quickly. They are totally coral safe but will eat any mobile invertebrate they can fit in their mouths. Small fish are in danger of being eaten by flounders as well.
Do your research on the species that you acquire, as some can get quite large.
~Michael