That is a tough choice. You have nailed down a lot of the weaknesses with the Spartan chassis and you are familiar with the rest of your Newmar. That is a well built coach that rides nice and has a lot of comfort. However, it is now (as of today) 20 years old and there is an infinite number of things that can go wrong.
The ISB is a proven workhorse and very reliable. That will be the least of Problem is with the new coach, there are a lot of things that can go wrong, meaning problems that were installed at the factory. Any one who frequent this forum know the manufacturers of RVs don't spend a lot of time making them perfect. So, you will have a long learning curve with the new one, figuring out how to fix factory problems before you are comfortable with it.
Do you prefer the devil you know or the devil you don't know?
Also, don't forget the new one comes with a feature the old one doesn't have, a monthly payment.
Good luck with your choice.