The fastest one takes a lot more amperage, so you likely will have to spend more on amps. Since the armature resistance is lower by the same proportion as the amps are higher, it seems that the faster motor actually has fewer turns of a larger wire. This makes sense, as then the back emf will be lower at a given rpm, and you will thus be able to turn faster for a given voltage.
Did you check with them what the price is? You might have to rethink your approach given your budget.