I have rollers and a trainer. Trainers offer more resistance on a less natural platform. That being said, I am only now getting to the point where I wish the rollers had more resistance - my avg speed on a 30-40 mile hilly ride is about 17-18 mph. Judge for yourself how that compares to you.

Also, many roller set ups have wind resistance add-ons so you can have the best of both worlds: a high resistance roller that forces you to think about spinning, balance and holding a steady line.