Both Newmar and Monaco are great coaches. However, the Cummins ISL is an 8.9L engine and the Cat C7 is a 7.2L engine. This may sound like a small difference, but a much better way to size up diesel engines is by the torque rating. The Cummins ISL is rated at 1200 ft lbs and the Cat C7 is rated at 860 ft lbs. An even better way to evaluate them is to look at the torque just above idle and the Cummins ISL will have almost twice the torque of the Cat C7. This means pulling away from a stop or climbing a hill the ISL will pull the load much better.
I would expand your search to not only the Windsor and Dutch Star, but look at the Endeavor, Imperial, Diplomat, Cayman, and maybe even the Dynasty. The key is to find the best coach for your money. Times are tough in this market and there are some outstanding deals available. The more you look the more you will learn and you will get a feel for pricing and be able to recognize the good deals.