I have both and both serve different purposes:
If I am going all out, intervals and such, I ride the trainer. I am sweating and breathing too hard to concentrate on keeping an exact line.
If I am working on pedal stroke, cadence, or general cardio, I hop on the rollers. This is a good workout, but I have to concentrate pretty hard to stay on them, as I have only had them about a month.
Good Luck!
RT