You can look at it either way....

ROLLERS
Pros - mimics riding on the road, works on your balance, works on coordination, smoothing out pedal strokes

Cons - Balance!!!!!, can't really take your attention away, can't really sit up or look around too much, some cheaper ones don't have much resistance


TRAINER
Pros - you can do pretty much anything in terms of training on a trainer, you can sit up and move around, you can't fall over, you don't have to keep a sharp focus on your balance

Cons - you're stuck in one place, some extra torsion on your frame, doesn't equate to road riding