In a perfect world I would have both. Rollers really make you focus on bike handling skills. You have to be able to ride a straight line. Your peddle stroke has to be smooth you have to keep your upper body calm etc...

With a trainer, you can do interval work easier to build strength. It's much easier to stand up and hammer to improve climbing. You can focus on blasting your legs and not worry about riding off the side when you're really tired.

I guess it depends on what your priority for training is as to which way to go. I currently have a Kurt trainer that I use with DVD training videos. I would like to add a set of rollers as soon as cash permits.