In the SP101, I think the difference is just over an inch. That's just past a third more barrel. I know it will make some difference. The obvious difference is in look, feel, and sighting. Exactly how much ballistic or handling difference will there be? I don't know how much this plays out tangibly but it looks like you pick up around 150fps
I know I generally prefer the feel of a 4" barrel. The idea of tapering is also limited by the distance between muzzle and extractor rod housing, making the slope more abrupt in a 3".