I use fluid. Rollers require much more concentration. I have small children and sometimes I am watching them while training. With a regular trainer, I am not going to have a roller mishap. The fluid ones are definitely much more quiet.
rollers will improve your bike-handling and balance as much, maybe more than they will improve your fitness.
I got tboned by a cyclist hard enough to turn my leg black and blue and green, but didn't go down. I attribute that, in part, to many hours on the rollers.
Rollers can be hard to ride, especially for the novice cyclist, and I find I'm more likely to go numb in various places due to a more static position. It is *very* difficult to stand up and shift around when riding rollers.
I have a fluid trainer but am currently researching rollers as well. I need to mix it up this winter or I'll get bored with it quick. The challenge of learning to ride rollers is appealing. Not to hijack, but does anyone know of a training video/DVD for a roller workout? I have a couple Spinervals, but is there anything geared more toward rollers?