Here are a couple of thoughts.... larger diameter rollers are easier i.e. require less power to spin. Typically, rollers are used for working on pedaling form and increasing cadence. To add resistance, one must add a fan or flywheel. A fluid trainer with a flywheel simulates the road better and will offer a better workout over the entire spectrum of power / heart rate levels. Rollers = z1/z2 Fluid trainer = z1 to z7.