I agree with Vmango that even a regular WRX hooks up really well in ice and snow. With the LSDs, the STi should be even better.
I'm sure with a experienced driver, the LSDs in the STi would result in higher handling limits too. However, IMHO the open diffs in a regular WRX might be a little more forgiving for most drivers in slippery conditions. My logic here is that in theory only one wheel should ever break loose at either end leaving the second wheel on each end to keep the car from sliding around too much.
I say this because I remember the first time I drove a truck with a locking rear diff in snow/ice, I found it to be much more tail happy than older open diff trucks I've driven.
Perhaps others would disagree