I'm not fortunate enough to be able to buy either but I am familiar with them. I spent a week at Newell spending time learning the ins and outs of their construction. I've test driven them and done a pair of articles for both FMCA and RV Magazine. Both the Prevost bus conversions and Newells are both great coaches but if I had the money I would go for a Newell without a second thought, regardless of which one was more money.
They both are a joy to drive and a good converter will do an excellent job in that the fit and finish and level of amenities will be up in the stratosphere. But to mne what separates the Newell from the Prevost conversions is the fact that they are purposely built. Each and every single frame is laid up from scratch to match the coach that the buyer is specifying. Nothing is adapted or converted from a bus and then made to work for an RV. The electronic computer controlled shocks result in a handling and ride that is amazing. Just by twisting a dial on the dash you can change the ride and handling from a cushy soft ride to sports car handling - at least as far as you can make a 50,000 lb vehicle sporty. :B
The way the plumbing lines are cut into the floor and insulated, the way the slideouts are integrated into the chassis, the overall construction is just plain impressive. Every little detail is put in perfect order and the result is a custom coach that really suits your needs. An example of one feature I like is the generator set. It';s a 20KW Martin genset mounted right up front. It doesn't extend on a slide but has hinged access doors into a soundproof compartment so it's easy to access for service. Yet there is no radiator up front. Instead, the coolant is pumped to a radiator and squirrel cage blower mounted in a driver side mid-coach compartment. This eliminates the open air access common to most gensets. The exhaust is further routed to the rear of the coach, along with the AquaHot exhaust. That's just one example of the many well engineered ideas that Newell uses.
There is another thing but this is just my opinion so take it for what you will. I like the looks of the Prevost H45-3 but I don't like the lower driver's position. I like the flat deck of the XL series Prevost because I'm back up high where I want to be but I'm not fond of the outside looks. With the Newell you get great exterior looks and the higher seating position. Again, those are just my impressions. Your's may vary.