We own a used Montana, but have looked at Grand Design units also. I'm not going to get into the almost religious arguments about which is better, but I will state that I'd NEVER buy either brand without an extremely thorough inspection. That means by us (not the dealer), with slides in and out, with water and checking for leaks, and every single electric or not appliance working. Next to no one buying any brand does this today - they get all caught up in how it looks instead and listen to a sales person telling them how everything's been checked out. NO manufacturer can guarantee everything will work on every unit they ship. That's a dealer's responsibility (although they'll get something back on warranty possibly), and it's up to you to FIND it before you take off down the road - because once you leave the lot, they can claim you did it!
Find the floor plan and amenities that fit what you need and like, and then find the unit that won't make you sorry you bought it.