When speaking about vinyl plotters, stepper motors are used in the lower quality machines. Servos, by far, are a better component - and as Fred states, much faster and accurate.
The problem, in regards to plotters, lies in the fact that when a manufacturer is looking to "cheapen" a machine (via the use of steppers), the also use many other inferior components.
Undoubtedly, you have seen the stepper machines on EBay. They are pure crap IF you have any intentions of providing quality work for hire. A few people here have them, and most do not like them (the machines) and their issues.
Expect to spend no less than than $1500 for a quality entry level machine that will give you decent service. The cheapest full featured machine (not entry level) entry level will start at $1700.