OP, all I can say is I envy your quandary :)
Of course, the Prevost is a more luxurious coach but a 10-15 year newer Newmar makes it a tough call for sure.  It seems like a bit of an apples and oranges debate not only for the original purchase price but because there will statistically more repairs (and more expensive repairs) with the Prevost.  10-15 years is a lifetime with respect to components inside an RV or bus getting rattled down the road.  I'd personally lean towards the Newmar just because I really hate dealing with things when they break.  The money to fix them and the time spent dealing with it are about equally frustrating to me.  But that's just me.  Everyone has different priorities.    
I would also be sure to shop other brands maybe in between the two.  Country Coach, Beaver, and Newell would all be on my radar. You are definitely into the upper end of coaches with a Newmar and Prevost but you kind of have the bookends of that class so to speak.  A top of the line Newmar is kind of just breaking into the ultra-high class coaches and Prevost is at the top of the list.  Lots of meat in between.