BMW 135 for the win. Twin turbo in-line 6, can't really beat that, ever.

The most attractive thing about the GTi in my mind is the turbo. You can ditch the intercooler and all of the excessive plumbing, make use of the obd2 mas flow to create a 3d map which will give you a good lambda reading, and run water injection (which allows you to run on less fuel on the freeway and run 91 octane to ridiculous CRs, something like 16 or 17:1). By eliminating the intercooler you make boost faster, pushing your power band further down, the fact that the GTi makes over 200 ft. lbs. of torque stock is a big advantage if you know how to use it. The GTi may weigh a lot more stock, but then again, what kind of person tunes a vehicle without at least putting it through a weight-loss/stiffening program?

The Civic Si looks only marginally better, I'm not a big fan of the way either cars look, to be honest. I don't know what Honda was thinking when they went with the new Civic design.

However, if we're talking purely off-the-lot, I'd take the Si. Because it's way easier on the pocket book and Hondas are always really easy to fix and find parts for.