I bought a used WRX once and got extremely lucky. I got an incredible deal on a low-mileage, well-maintained, unmodified car without really realizing it at the time. The car served me faithfully for the 3 years I owned it.
Knowing what I know now about how many owners aren't as scrupulous with their cars, I will never buy a used WRX/STI, unless the price is low enough that I can factor in the cost of a total drivetrain replacement within a few years (and even then, I'd be extremely cautious about the car I chose).
Personally, if I can't afford a new WRX (or really any performance-oriented car), I'd buy an entirely different type of vehicle.
Other than that qualifier, I would prefer to pay cash for a couple year old car, rather than take out a loan for a brand new one.