There are several threads about this already if you want to do a search. I actually have both and I think rollers are way more fun. The trainer is really really boring-- worst than the stationary bike at the gym. You can get DVD's to make it better but personally I think it's a device of pure torture.

On the other hand, I just got a set of rollers and I think they are more fun to ride. In the beginning, they require practice and concentration but they just feel less boring. Both the trainer and the roller offer different workouts. Rollers help you with your balance and pedal stroke but they won't help you climb any hills. Because you can change the resistance with a trainer, it can help build up strength and power.