The 4.5" rollers with the fan unit is what I have and highly recommend. If you don't want to run the fan for noise reasons (it's not _that_ loud, but loud enough), then I would recommend the 3.0" rollers with the inertia unit as a good alternative. You'll be pushing almost 225 watts at 20 mph, which is pretty realistic and the flywheel will allow a longer coast-down and more realistic feel when accelerating.
The 2.25" rollers aren't any harder to ride in terms of technique, but they do add a lot of drag. 20mph with the smallest rollers and intertia unit would be about 300 watts, which is more than most people are going to want to push. This means you have to ride slower, which can be a little annoying in terms of balance.
Two things that are nice about rollers is that, by themselves, they're super quiet and they require much more attention than a trainer. They can actually improve your riding technique where as a trainer allows it to degrade a little (IMO).