Richard,
The chassis of your motorhome is very important. Both Spartan and Frieghtliner will work just fine. Just make sure you get one with sufficient length and carry capacity for you to full time. Do not rely on any of the three makers you mentioned to put the chassis you need beneath your motorhome. Look up the specs. Make sure you have enough carry capacity when fully loaded to pull your towed and accept all the things you will carry and still handle with comfort and not cause you to white knuckle on the highway.
The last time I looked at motorhomes there were many many models of many lengths. If you look closely you will see different chassis lengths and weight carry capacity depending on the model, length, and perhaps what you order special. Keep in mind that the less front and rear overhang you can get makes for happier time behind the wheel. Short chassis means you will spend endless hours keeping it between the lines. And that is with or without electronic add ons like comfort drive. The best chassis is the one that goes down the road without anything to help it. Then if you add a feature like comfort drive it will add to an already good system.
I don't own any of the three you mention. I do like good vehicles and do go to shows and have many friends that own motorhomes. I would not rank Newmar over Entergra. They are both mid to higher level coaches that in my opinion are quite similar. Likely if you spend X amount of dollars at either you will get very comparable features. Likely similar motors, transmissions, quality level of interior and so on. I am not very familiar with Thor and cannot comment.
The several people who have posted the importance of floor plan have it right. It is very important you love the floorpan. If you don't love it, keep looking. Tiffin, Winnebago, Monaco, and Fleetwood also make motorhomes of comparable quality and cost to Newmar and Entegra.
I watch this forum and also IRV2.com. IRV2 has brand specific forums. If it were me looking to buy a motorhome I would go and watch those brands for a while to see what other people are saying about those makers.
By the way. The best chassis is very likely a Prevost. That is a very high end bus chassis good for 2 million miles. Below that are the chassis made specifically for that particular coach by makers like Foretravel, Country Coach, Monaco, Travel Supreme (Who was bought by Entegra.), Wander lodge, Newell, and a few others. I believe Entegra still puts a custom chassis under some of their models. And I think Tiffin puts the Peak chassis under a few of their coaches which is a custom chassis. Monaco I believe makes some custom chassis.
But back to your question. I would not worry about the two brands of Spartan or Freightliner, but I would make sure they put a big enough one under your motorhome.
RFCN2
Country Coach
Jeep Wrangler Unlimited