I have both. The 10.5cm Nikkor-P (sonnar) is a fine lens and contributed to the making of Nikon's early reputation in glass. It is softer. Sometimes that's nice. But I will stand and testify that the 105./2.5 Gauss (I have the Ai but would love the AIS for its shorter focus throw and hood) is one of the three or four finest lenses I've ever used. (Some others? Zeiss 50/1.5 and 50/2 sonnars, Zeiss 50/1.4 planar for C/Y mount, 85/1.8 Takumar, Nikon 180/2.8 ED, the humble but astonishing Rokkor-X 24/2.8, and, ok, a couple of Leicas which I can't afford to own.) It just bowls me over sometimes, the look it gets. And the colors. And the sharpness. And the bokeh. So I'd go with the AIS, a clean copy (pay more if you possibly can) and you will never ever regret it.