Had the Cusco 2 way on my old MY05 and it was a fantastic diff. But as everyone said it is clunky as all hell at slow speed corners. i.e. you get funny looks at the shopping center.
I now have a STi 2 way in my MY98 and it is the perfect unit for a daily driver imo. Easy to drive, no noticeable extra noises.
One thing I have noticed is with the Cusco unit, I would get mega lift off over steer (probably a lot of tank slapper videos of me on youtube
) where as the STi unit doesn't have this effect (feels more like a 1.5 way almost). This is the biggest difference imo and if the car is more road going, I'd pick the STi unit as it is hard to keep the foot in to avoid the lift off oversteer if you have an oopsie on the road. If you can fix that link between the brain/foot... I'd go the Cusco unit as it just felt so impressive to drive.