I would echo much of what has been said. I have had both a NAS 90 and a 110 in my garage at the same time on two occasions. The first time it was a NAS 90ST and a NAS 110, and second was a NAS 90ST and a ROW (NAS110 interior and exterior + pursuit 4.6 moved over to a ROW frame). I ended up selling the 90ST on both occasions because while I prefer it in many ways it doesn't fit my whole family (3 young children).
As others have said - I think the 90 and 110 serve different purposes. The 90 is more nimble and fun around town, especially the ST. The 110 is better on longer drives, trips, and better for year-round driving (I daily my 110 9-10 months of the year in the northeast). While the NAS 110 didn't feel much slower than the NAS 90, my current 110 with the 4.6 is night and day - more responsive, better on the highway, etc. Also with my current 110 I can drive the crap out of it without worrying about it - like I did on trips to Baja and the Romp (both in the last 4 months). I actually traded my NAS110 for my current 110 for that reason. I wanted to drive it, as much as possible, and not worry about the NAS premium.
Is your 90 an ST? I think a NAS 90SW and a NAS 110 would be redundant. Yes, there are differences as mentioned above but I would either pick one or the other. In my ideal world I would have a NAS 90ST for around town and a ROW 110 for everything else. Now I have convinced myself that I need to buy another NAS ST....