Simply put, the Subaru WRX is quick, while the STI is even quicker. But there are plenty of other differences in the STI that make it a worthy upgrade for focused enthusiasts.
Both models share the same 2.5-liter displacement from an archetypal Subaru flat four-cylinder engine, but the STI's sharper tuning and Driver Controlled Center Differential (DCCD) extracts the most from the powertrain and the chassis. Between sedans and hatchbacks, all else the same, performance is equally rewarding.
In the WRX, its 265-horsepower output enables quick acceleration in nearly any gear of the five-speed manual transmission. The STI's 305-horsepower rating is even more fun, though, and is paired with a six-speed manual. No automatic transmissions are available--as it should be in what are performance cars.
Behind-the-wheel personalities, and even the sounds these two models make, are quite different. The WRX's exhaust note is throaty, but muted. The STI's, on the other hand, is more raw and omnipresent. Depending on the mood of the driver and the passengers, that can equate to more enjoyment, or more annoyance.
The STI's willing partner is the DCCD system, which sends 41 percent of torque to the front wheels and 59 percent to the rears. There's an Auto, Auto+, Auto-, and six levels of manual center-diff lockup, all accessed through a little flip switch in the center console. There's also an SI-Drive controller, with Intelligent, Sport, and Sport Sharp modes available. Each makes throttle response progressively more direct, while the DCCD system allows the driver to bias power rearward when the situation requires.
The tability control system in the STI also includes a "traction" mode, which continues brake use but eliminates throttle cut to improve performance. For the maximum in driver control, the system can be disabled completely. Larger Brembo brakes on the STI enhance stopping power and fade resistance, aided by the Super Sport ABS system, which helps to reduce understeer.
Whether you opt for the WRX or the STI, both have tenacious grip and poise and are particularly impressive in the transitions. Of course their front-heavy weight distribution and suspension design contribute to the potential for understeer, but if you dab with your right foot--and this is somehow a bit more fun in the WRX--you can neutralize it and slingshot out of a corner with the AWD system scrabbling for more grip than you knew was there. The STI is faster by the stopwatch though, and adds unique springs, bushings, stabilizer bars, and ride height, plus a pillow-ball bushing setup at the steering pivots.