I owned an 03 SVT Focus and switched to the MS3 last year. There are some + and - to the MS3 from my perspective. I used to run the SVT at 4 track weekends and a couple of AutoX every year. Handling the clear winner was the SVT Focus. However, it doesn't have a limited slip which really hurt in low speed turns and on really fast sweepers where your pushing the edge of traction.
I've had people come off of track and ask what was under the hood because I could come off a curve at a higher speed than some true sports cars. For the money, I don't think anything out handles the SVT. Powerwise, my SVT was mostly stock, but had throttle body, underdrive pullies, reprogram, and ram intake. It probably made about 155 at the wheels.
The weight difference (2850 vs 3150) is very noticeable in transition turns form side to side like s-curves or slalom. The SVT responded better to weight transitions. On the track the SVT would out brake the MS3, but it reverses for street driving.
Now the good stuff for the MS3 - Power. Where the SVT might be able to carry more speed through some turns or s-curves, the MS3 comes out of a curve like a car with more power. The torque and limited slip work to make the car come off an apex like no fwd has a right to.
The MS3 is more vague on track, but once planted, does very well. The back end does like to move during turns though and it can be unsettling.
I've seen a lot of SVT Foci with power adders and none have been too reliable. But if you could get to around 240hp reliably, I think the Focus would be the better performance value. There in lies the issue though. When I weighed it all out I bought the MS3 and while I have mixed feelings about the comparison, I am very happy with the MS3.
The fit and finish on the MS3 are worlds above the SVT Focus and I do think the Focus doesn't get the respect it deserves from the anti-american car crowd.
Daily highway driving the MS3 wins hands down. The power to pass on the highway is very impressive. The handling is good, but the handling/ roughness trade off isn't as good as the SVT. It can be a bit jouncy on freeway expansion joints when compared to the SVT. The MS3 is noticeably larger on the inside and as a result is more comfortable.
Premium stereo on the SVT is far better than the premium Bose on the MS3. The Bose really doesn't have bass when compared to the audiophile option.
Gas Mileage - I average 25mpg on the same commute that I would average 29mpg in the Focus.
Insurance went up about $75 per 6 months - probably just because it was a newer car.
The MS3 is the SVT Focus Ford should have built, but didn't. My advice is to do it as long as you can live with some trade-offs.
Have more ideas, but I'm late for dinner (drive2)
PM me or ask here with any specific questions and I'd be happy to answer.