Baron, I'm one of the regs at SB^2 that drives an STi. One of my friends in Burlingame drives a C43, and i can tell you that when he rode in my car on 280, he said
"*** this car is so much stiffer than my C43, it must handle pretty well huh?"
then i took him to hwy 9:
"uh...aren't you going in a little hot??....!!!....damn"
ride comparisons are pretty radical between the two. The Sti and its agressively bolstered seats (compared to C43) and a lack of that plush leather means that the seat is pretty stiff compared to the C43. If you were barely able to tolerate the stiffness in the C43, you won't like driving the STi everyday to work.
The noise levels are also pretty different. I rode in his car with the radio off and as expected out of an MB, its fairly quiet, the STi on the other hand, will let you map all the bumps on the road just by feel and sound
Power diffrences are basically the differences between the drivetrains and the width of the drive tires. The C43 doesn't feel like it has as much push off the line as the STi because its rwd, but once it gets up to speed, the STi has a bit of drag from the spoiler and the awd drivetrain.
One last important difference, you'll be shifting quite a bit in the STi, while its fun for most enthusists, if you'll be driving around in SF to goto work, its gonna be a real pain in the butt.
C43 is an excellent all-around sport sedan, the STi is a hardcore jolt-you-to-death sports sedan, if you can tolerate all the raw noises and the large amounts of tactile feedback, the STi will reward you with a piss in your pants ride from start to finish.