I didn't see any meaningful difference in sharpness between the f/1.8 and f/2.5. The real differences are that the f/1.8 handles flare much better, much less CA, and lacks the prominent corner curvature of the f/2.5. At very close ranges the f/2.5's curvature and CA make it's performance look quite poor compared to the f/1.8, but pictures of most subjects and normal ranges typically do not show these weaknesses.
The f/1.8 certainly has some flaws, the bokeh is not as nice as the f/2.5 Ai and can sometimes be too busy for me. When the focal plane is at a strong tilt to the subject, the bokeh can get a bit barfy too and really must be kept in mind. The f/1.8 can also be pretty susceptible to aperature reflections, which can either be a joy or an irritation depending on your preferences.
Seeing that the 105mms are generally seen as portrait lenses, it is easy to see why the f/2.5 is more popular by a long shot. In most portraiture use, the advantages of the f/1.8 simply are not present while the size and bokeh disadvantages are very noticeable. I prefer the f/1.8 for my uses, but would choose the f/2.5 Ai over it for portraiture without hesitation.