I got this in an email from the president of a well-known converter:

Prevost bus conversions without the ISS are still far superior to anything out on the road as for handling, etc. I would challenge anyone who can drive a 1996 without ISS and then drive a 1997 or 1998 with ISS, and without actually looking at the date of manufacturer or underneath the coach at the suspension to tell me based upon driving which has ISS and which one does not.

So there you go.