Before buying the Mazda3 I test drove a bunch of cars such including the Impreza TS and RS, Corolla, Civic. My second favorite behind what I got was the Impreza. It didn't feel quite as quick or handle quite as well as the Mazda3 but it seemed like a definite step up from the others. Maybe I was having a hard time being objective though because I really wanted the Mazda3 more than everything else.
Weight distribution for the Impreza is 59/41 for the Mazda3 is 62/38 I think.
Other factors for me were I could get the 3 with side air bags and a moonroof, but I couldn't get the Impreza with that. If you get the side air bags with the 3 you also have to get ABS, in the Impreza ABS is standard. I like the 17in allows that come with the 5dr (opt. on 4 dr) a lot.
As far as inside/outside looks go I liked the 3 but don't take my word for check both cars out and decide that one yourself. Really I'd say the same thing for performance as well. Take both cars out for a long test drive w/out any salespeople in your car and see what you think yourself.
As far as reliability, what djcg posted looks scary. I was surprised to see that cause I've heard a lot of good things about Subaru as far as reliability (know a lot of people with them too because there were a ton of them where I use to live). The Focus is up there high on the list, but supposedly it is a lot better now than when it first came out. It was hard to tell from that list what years the Impreza was bad or if it was bad every single year -- that's important to know since it's been out a long time. The Mazda3's reliability is unkown at this point of course. A lot of people tell me it should be good since it's built in Japan but I had a Camry that was about 6 years old for a year and I had more troulbe with it than I had with a 15+ year old Volvo 240 for 4 years and the Volvo had more miles.
