I own both and use them for different reasons and situations, such as:
- More realistic feel, albeit less resistance.
- Require attention - you can steer like on a normal ride, and if you lose attention, can steer right off! (You usually only do this once, after which you've learned a good lesson to pay attention ever more).
- Great for recovery rides where you don't want to push too hard.
- Great for spinning
- Great for learning to balance and smooth your pedal stroke. Bad / uneven pedaling shows up as lateral movement, so you learn to round your stroke and get more efficient.
- Great for checking bike fit with regards to cleat position and saddle height - much like on the road except with out the distractions of the road.
- Hard, resistance training to maximize workout in compact time. I rarely spend more than 60 minutes on the trainer.
- Sprinting and intervals - you can get out of the saddle and push hard without worrying about falling off, like on rollers.
- Mindless ride or recovery - sometimes you want to recover or ride the legs out without thinking, so pop on the trainer and put a good movie on TV.
- Race warm up - rollers are good, too, but trainers require less space and are a bit better at uneven ground.
If you can only afford one right now, get the trainer. But save up and buy rollers and then you'll be able to enjoy the benefits of both.
Oh, and once you master rollers, try making breakfast while doing them! Like this: http://www.bikeroar.com/articles/breakfast-on-rollers-meet-christy-red-rocket-keely.