On a trainer, you can do more power training. It's more difficult to simulate a long climb or a sprint (standing or out of the saddle) on a set of rollers. And, in the case of a Fluid2, you can really produce big power...

Rollers are great for smoothing out your pedal stroke, improving bike handling, etc...

If you are a crit racer and want to simulate final sprints, or want to do extended standing climbs, that's really not what rollers are for. In other words, you'll get stronger or more powerful, faster, on a trainer. You can develop a smoother stroke and better balance on rollers.

Really, if you can, having both is the best.