i have both. I live in Ontario, Canada, where the winter is long and inhospitable so riding indoors is a necessity. I had a trainer for several years and just got a set of rollers last winter.

I use the trainer for long sessions (over 1.5hours) or for intervals. I use the trainer for long sessions because I can watch a movie (to distract myself from the fact that I am getting nowhere fast) while on the trainer and for intervals because I can easily adjust effort. On the trainer, I would for sure get an indoor-trainer-specific tire (hard plastic, no traction on the road but really durable for the trainer).

I use the rollers strictly for spinning and for warming up (before doing strength training, etc.) and not usually for more than 1hr, maybe an hour and a half. I say this because the rollers (at least for me anyways) need more concentration. I cannot watch a movie while on the rollers, or have my children around otherwise I am at risk of losing balance and falling off, It's strictly the ipod and my spinning legs . I also find it harder to do intervals on the rollers for the same reason, risk losing balance when changing speed, standing up... The rollers are A LOT of fun though and great to develop/maintain your core. I credit the time spent on the rollers last winter with better handling in slippery situations (ice, mud..).

hope this helps. good luck in your shopping around
julie