As with most comparisons with these two, things start under the hood. Ford has claimed the Focus RS makes 350 horsepower and 350 torque. On a Dyno, 282 horsepower and 328 torque managed to meet the road. Subaru has claimed the WRX STI makes 305 horsepower and 290 torque. On a Dyno, 247 horsepower and 243 torque managed to meet the road. The figures seem odd for several reasons. The most obvious is that manufacturer claims are usually different from what the cars actually produce. The other element to consider is that there will always be a loss in power from the measurements at the crank when compared to the wheels when it moves through the driveline. However, with flubbed figures from both manufacturers, it's less important to look at the loss in power and more useful to see that the RS has the STI beat in this area.
Part of that performance comparison gets gray depending on what you like to feel. Where the RS is using electronic power steering, adjustable suspension, and video-game drive modes, the STI is running heavier hydraulic power steering, tuned inverted struts, and a Driver Controlled Center Differential. That modern technology in the RS edges out the STI on the track, but the feedback from the drivetrain to the driver makes the driver feel more connected and confident in the aging Subaru. It depends on how you'd like to drive the car: with a hammer or a XBox Controller.
These figures have been the source of a countless debates. Some STI owners will defend that their car is still better to the bitter end, but the numbers don't lie. Performance-wise, the Focus RS has the STI in nearly every track-based test and that says a lot about both cars. On the one hand, there's really no denying the Focus RS has the WRX STI beat. The technology and components of the Focus RS are nearly 15 years newer than the dinosaur found within the WRX STI, so we can knock the EJ (pun intended) all we want but it has seemingly aged very well. A 45 horsepower gap and years of automotive development ahead of the STI really should have the Focus RS out front, but it's certainly not a landslide victory to brag about. Both cars are still properly quick.
While there's more power to be had with the Focus RS, the WRX STI is still on a bit more of a diet without compromising space. The Ford Focus-based RS is heavier and still somehow smaller than it's rival. The Subaru Impreza Sedan that the WRX STI is built around has more passenger room inside as well as higher crash safety ratings. Both have nearly identical wheelbases and track dimensions but the hatchback offering does make for more versatility for loading larger objects in the back of the RS. If you've got the seats down more, the RS will serve your daily hauling needs a little better. When those seats are up, the WRX STI will be roomier and safer for your passengers. And while neither the STI or RS have a glass-smooth ride quality, those passengers will appreciate the more forgiving ride in the STI, too. Even in normal mode, the Focus RS can be a bit of a bounce house.