Here is the opinion from someone behind the wheel of both, owned both(still does but different generations) -
STi is more fun than should be allowed legally in a car. If you own one there will be two certainities.
1. This car should not be legal.
2. The rating of 300HP and AWD do not compare to 390HP RWD. They are different beasts.
It should be noted this is my first venture straying from the Mustang brand into the Japanese auto world. While I'm still a devoted Mustang fan, after driving GTs, Mach1's, etc... The Mustang is a great car but a different breed. There have been write-ups showng the STi outperforms the Cobra overall, and agreed when going above 130 MPH the cobra probably would pull away(whenever you need that). I've been to 110 in the STi and with no noticeable desire to stop pulling, lots of throttle left. What caused me to drive and buy the STi is the sheer feeling of control, power, and the inability to do no wrong. Likewise the same things that scare the $hit out of me when I'm driving it.
When it comes down to it - a large portion is preference. For me with kids the back seat of the STi was desireable, 4 doors and you can hang or beat most of the "street racer"
op) guys out there. I test drove the STi and fought the urge to buy the STi tooth-n-nail for 2 months. I even had a Mach1 on hold, but I had to have the STi. Still feel a little guilty about going Japanese, but it was the right choice for me.
It really depends on how you are going to drive the car. I LOVE the look of the '05s and may have the desire to switch back at some point. But the switch point is for the style and my history with Mustang, I seriously doubt it will be the technical merits compared to the STi. Its always best to get many opinions and hopefully of some that have had seat time in both types. Other than that its just as useful to read print on the bathroom walls looking for girlfriends, if they really haven't been with them how do they know. Chances are they weren't there and didn't know or just want to trash somebody. I'm impressed a decent amount of the writeups are not too biased.
I could not be more honest when I say the biggest concern with the STi is the trouble it will enable you to get in, the car is SO much fun to drive and such a feeling of control its unmatched to anything I have driven. Now if you can afford twice the sticker you may be able to compare. The car hugs the road like its on rails, IMO is best suited for daily driving with some fun in between because of that fact. You can buy a 1/4 car and be afraid to turn(really great going straight) or buy a car that feels like its one with the road and fear nothing. Double edged sword though.
Now since I've had the car for 6 months - I do fear the car, mostly because I have not seen its limit really and know if uncorked it could crank out 475HP and still be street-able in AWD. The additional costs when you buy an STi -
1. Radio(I purchased gear and still have not installed it, the spool up is still keeping me giggling like a 10 year-old on a go kart).
2. Winter/All Season tires - I did run my Potenzas and BBS's all winter but no snow(too anal then), Most do buy a set of Winter/All season tires and rims(subydude.com) to swap(take advantage of its winter potential - AWD). The potenzas are sooo sticky in AWD, half the time it feels like your ripping up asphalt. Hehe.
The styling grew on me, it was the drive that sold me. When someone talks about turbo lag on an STi, they haven't driven one or had enough seat time. My analysis after owning both styles(Stang - 8cy - RWD, STi - Turbo4cyl - AWD). An 8cyl. has a wide power band, its flat out easier to drive since you can know little about the engine characteristics and keep it flying. The 4cyl. usually has a sweet spot, stay within and hold on. I've been in Turbo powered cars many times and if the size of the turbo and the displacement of the engine get too great in deltas(difference in potential of the two) the lag is annoying. The STi has extremely little IF ANY noticeable lag, Power kicks in at 3k, comes alive at 4k, slams you back in the seat at 5k, and at 6k your looking for the POlice. With the 6 speed you can keep it in the power band all the time once you learn how to drive it correctly. My engine broke in at around 1200 miles. I thought the car was just wrong to own prior, once I hit 1200 miles it would spool up on a light breeze of me stepping on the pedal.
Final notes - Mustang has the style, that if you love it will not be matched. The 8cyl. is great. The RWD is cool if you want the feeling of the car breaking loose on you, but not unlimited control by any stretch.
The STi, 1st drive is addicting, the more you own it the more you realize the car should not be allowed to purchased. I have not modified my STi at all yet. May never? The correct combination of displacement, turbo, AWD, HP and Torque curves, 6-speed manual, DCCD(Driver Controlled Center Differential) unreal. It is just a really well designed car, hands down. For anyone who is not familiar with the DCCD, you can adjust from within the cockpit whether to let the computer distribute power to all four wheels or over-ride distribution between the front and back. I like this feature quite a bit, because if I want the feeling of RWD I just distibute(thumbwheel switch) power to the rear and it feels RWD. You can distibute power all to the front(don't know why, depends on racing conditions though), or some where in the middle. If you want to 1/4 mile it, you can lock the differentials from the cockpit and launch like no other. I have not felt a launch like that since a heavily modified 350(yes Chevy) motor, M-22 4-speed Muncie, and 12-bolt posi-rear Camaro. The STi lauch is unreal, you might chirp them a little bit, but you won't waste time standing still - the car is GONE! Hope this helps, its a tough call. We'll see when the '05 Stang comes out what you think. I really hope they bring the 5.0L 3v to production in the Stang that would be nice! The pics are beautiful, well designed visually. A friend of mine saw one('05 Stang) front and center at an aviation show(Ford exhibit at an aviation show, hmmm) last week in Oshkosh. Wow! Now my wife wants one instead of the SUV.