The impreza wagon was one of the other cars we were looking at when we bought. We also looked at the Saab 9-2x which is the same thing as the impreza with minor changes. I'm assuming you've test driven the Subaru and the Mazda at this point, right? Well I'll give you my thoughts on them, and anywhere I thought the saab was different I'll throw that in too.
Power: Engine wise you're looking at a whopping 13hp difference for 0.2L of engine size. This is after a small power increase for 2006. The '05 I drove didn't feel much different between them, but then there was only a 5hp difference. The Impreza is about 200-300 pounds heavier so this might be what offsets the power gains. Just want to throw in here that the gear box on the 3 is remarkably smoother and more comfortable to shift. The Impreza felt a little gritty and just didn't flow as well from one gear to another.
Handling: Well, the Sub. is going to hold the road much better than the 3 is due to the AWD, but overall the 3 feels much more nimble on the road, and more responsive. The more responsive steering in the 3 is probably due to the electro-hydraulic power steering and being lighter. The 3 can cut a corner a foot shorter than the Impreza despite having a 4" longer wheelbase. The Saab had a much softer suspension so was more comfortable while still feeling sporty.
Comfort: The seats in the 3 are a little stiffer than those in the Impreza so are more comfortable for 'spirited' driving, but can become a little uncomfortable on long trips. I've done road trips in a forester which stayed pretty comfortable overall. The 3 offers more space to occupants with the exception of front head/leg room where the impreza comes in with less than an inch more. The 3 gives more than 3" more of rear leg room however. I noticed no lack of good A/C in the Impreza, though I think the previous A/C problems in the 3 have been fixed. All controls were easily reached from a comfortable driving position. The 3 offers a tilt/teloscopic wheel vs. the impreza's tilt only.
Safety: Both cars come standard with ABS this year, though only Mazda gives you Electronic Brake Force Distribution (truthfully I don't know what this does, but I assume it helps you brake better. j/k ) Also both cars give you four wheel disc brakes, with the Mazda receiving bigger rotors. IIHS doesn't have any side impact statistics for the Impreza a this point, and the 3 was rated poor when tested without the optional front side/side curtain airbags. I believe that other similar cars that were retested with side airbags got much better results. Both vehicles received Good ratings on front/front offset. The Impreza isn't available with side or curtain airbags at all, though the Saab has front side airbags.
Economy: Between basic models you're not going to be paying too much differently for your new car as they end up being about $500 off from each other. The 3 gets better gas mileage than the impreza. With a smaller tank it's still rated for longer distances. Of course this all depends on your driving habits. The Impreza is also heaver which lends to it's unmiserly-ness. Mazda and Saab offer 4yr/50k mile across the board with Sub giving a 5yr/60k powertrain, 3yr/36k bumper to bumper warranty. The Saab adds around $2k to it's price tab for it's Saab badge and other minor modifications.
Functionability: Obviously the Impreza has AWD which will give pretty much anyone that feeling of invulnerability when driving in adverse conditions, plus about an inch more ground clearance. Beyond that the Impreza offeres nearly twice the total cargo space of the 3 while giving about the same for behind the seat space. If it makes a difference to you the Subarus come with a retractable cargo cover vs. the hard cover of the mazdas. I think the Impreza also gets a basic roof rack, or possibly just the side rails without cross members. Just as a place to add this, the 3 is wider and longer than the Impreza, though the Impreza is a tad taller.
Styling: Hands down the 3 kills the impreza here. The exterior and interior of the impreza is pretty boring overall. You don't get the feel of everything flowing together and being integrated as much in the impreza (though it should be easier to put an aftermarket deck in the Sub). Also the fake silverish trim on the dash on the Impreza can get worn and look pretty crappy after a few years. Most people that sit in the 3 are a little taken aback at first because it just looks so cool. The 3 gives you electroluminesent red on blue gauges with a red dash (didn't like it at first, but love it now) vs. the Subaru's greenish/yellow. The Saab has some different upholstery/door inserts which brighten up the otherwise dim interior of the Impreza, also the front bumper/headlights were replaced with Saab-ish parts to make it look more refined.
Toys: No doubt you can get many many many more options on the 3, though many of them are only on the Grand Touring which will run you a bit more. (xenons, moon roof, leather, TPMS, auto wipers, etc) If this is something you're interested in then look more toward the 3. Also add in the steering wheel controls as a plus for the 3. I love these and get really annoyed when I'm driving a car without them. If you're still set on the impreza but want other things like heated seats/mirrors, leather or a moon roof then the Saab is for you.
Overall the cars come pretty close to eachother so really I'd ask what is it you want from your car. The Saab adds a hefty price tag for not too many differences, though despite that we decided we would've bought it over the Impreza because those differences "made" the car for us. Drive them both and look over the standard/optional options and make sure you're going to be happy with your purchase. Hope this helps some for you, and as always questions are welcome.
~icspots
