The optical formula is quite similar. The AI has a longer focus throw than the AIS and some prefer that because you get an expanded focusing scale. When using my lens with film for street photography (and mostly zone focusing) I preferred the AI to AIS.
When tracking down my current lens I found that the latest serial numbers of the 28mm f/2 AIS switched to SIC coatings. Some have noted that this is creates an even better unit. My new lens is in this range but I haven't had a suitable test situation yet. One prior AIS was not and I did not notice any ill effects. I have had both AI and AIS lenses and I find the AIS lenses to be quite good and I like the shorter focus throw.
You can check the pinned posts (Ronald Vink) at the top of this forum for a reference to the SIC SN = 575001.