With cooler temperatures arriving and daylight becoming less and less I have cyclists ask me frequently should they get rollers or a turbo trainer. Both devices offer great benefits that will help you improve your fitness but the answer isn't totally cut and dry.
I think every cyclist should learn to ride rollers. For creating a smooth, fluid pedal stroke there is no better tool than rollers. Rollers magnify any irregularity in your pedal stroke thereby "forcing" you to become smooth. You can do relatively high heart rate stuff with rollers but a potential downfall is that it can be hard to do high power stuff. Some rollers have the compatibility to accomodate a resistance unit however rollers make a short, sharp out of the saddle effort nearly impossible because of the risk of coming off the drums.
I like rollers because they keep you engaged. If you stop paying attention you'll fall off. Additionally they are the most like riding your bike since you are, essentially, riding your bike on three rolling drums. This combination makes the time seem to go by faster and its more fun.....if fun is something you could say about indoor training. I also find that you can rack up quite a few miles on the rollers if thats your goal.
On the other side we have the turbo trainer. It makes indoor training approachable for everyone and doesn't require the skill that rollers ask for. The turbo trainer allows you to make short, sharp efforts without the risk of falling off. Many turbo trainers come with a resistance unit and they make power training very easy. Many new trainers interface with a computer and allow you to monitor any number of parameters. With power training being in vogue right now the turbo trainer has a clear advantage.
The downfall of the turbo, in my mind, is that it doesnt develop pedaling fluidity in quite the same way and frankly, its incredibly boring. With the bike locked into the device its highly stable and no finesse is required.
To me the answer is: depends on what your goal is if you ask which unit is better. In an ideal world you could have both. I realize that its not always feasible to have both though. If you need to work on developing power than the turbo trainer is the way to go. If your goal is to develop a fluid pedal stroke, reduce the monotony of indoor training and actively spin your legs, I think rollers are the way to go.
Everyones opinion will be different so you have to find what works for you. At the end of the day, as long as you continue with your training on the days with inclement weather or cooler temps it doesn't matter which device you use.