Its hard to compare the two cars. The STi is MUCH faster, with way way more grip. It also has more amenities, like a decent sound system, and a place where you can actually fit a CD jewel case.
I haven't pushed the STi handling wise yet, but so far it is very very solid for its size. The turn in is very sharp with minimal body roll. Ride harshness is about the same as the S2000. Steering is very fast and precise, I'd argue the ratio is close to the s2000.
It obviously understeers at the limit, compared to the snap oversteer of the s2000. But with the driver controlled centre diff, I can put up to 65% power to the rear, so I can step it out when I want to.
I said it before on another forum. The s2000 is a scalpel, the viper is a sledgehammer. The STi is a steak knife.