Which would you choose and why?  I know the Mazda makes more power and is turbo, but thats not the end all.  

mazdaspeed3.  for barely more than $2000 you get alot more car.  the power increase is obvious, the chassis comparison may be yet to be done, but the mazda's should be just as good if not better than the si's.  plus the mazda's interior is alot nicer, and not to stroke the dash or anything but that is where you spend all your time.  the new civic has not-bad materials inside, but the spaceship theme is a put-off for most people.  plus, anything with a k-series is guaranteed to be a theft magnet, it even says so right on the window sticker

the only thing the civic now has going for it is it has the new 4 door for those of us who need the utility, and much better gas mileage than the 3 (which is a really nice thing as well).