The STi has a nice blunted leading edge, longer blade length, and shallow....kinda Ping G like. Narrow heel to a widish toe
STi2 is more traditional looking shape, plenty of sole relief but more of a rocker sole vs. the blunted leading edge. Very good for steep swinger, easy to get thru the turf. Sole width more uniform.
I liked them both but the STi2 is an improvements in looks and feel. I did not care for the feel of the STi, was too muted along with it being very forgiving at times was hard to tell between center, off center, and slight mishits. Strange feel with those ball bearings. I got a lot off good use out those babies. Ugly little ducklings but quite effective!