I have a lot of respect for Newmar diesel coaches, but Entegra has a reputation for giving you more coach for your money. In my opinion a Cornerstone would compare better to a London Aire.
Just to correct some information, the 500hp Cummins in 2016 is the ISX12. It is an internally downsized ISX15 designed to replace the much lighter weight Cummins 500hp ISM. The ISM was a 2070# engine and the ISX is a 3093# engine. However, the ISM was not passing the new emissions requirements so the least expensive redesign was to downsize the ISX15. Still, they are all great engines.
I would be cautious with the Cummins 650HP ISX. If purchasing a coach with this engine I would budget $20k for the new cylinder head or address the EGR system.
Best of luck!
97 Monaco Windsor- Sold
07 Monaco Executive McKinley- Sold
04 Monaco Signature Chateau IV