I'll throw a monkey wrench into this whole thing, so be aware that my opinion is probably of little value.
I'm 5'11", weigh about 165 lbs, and ski reasonably well. I'm older, so I don't ski particularly fast and I don't do big drops. I lived in Colorado for over 20 years; Winter Park was my home area, so I'm quite familiar with MJ. I now live in Nelson, British Columbia, where a different kind of mj hangs in the air sometimes, and the snow is usually denser and deeper.
I own a pair of 177cm Mantras and a pair of Others-That-Shall-Remain-Nameless, about the same length, but with a 78mm waist. The Others weigh a whole lot more than the Mantras.
First, I'll admit that most of the deep powder crowd around here prefers skis that are bigger than the Mantras. In fact, many of them use reverse sidecut, reverse camber waterskis of various brands, regardless of the actual snow conditions. So, my use of a "little" 96mm ski on deep days immediately marks me as very uncool and of deeply suspect family history.
Still, the Mantra provides plenty of float for me, and I find them nimble in the trees and around the rocks, with a huge sweet spot. They're easy. They handle the heavy snow here quite well, including the out-of-bounds sidecountry that's legal and so delicious in Canada.
Of course, my perspective is affected by the fact that I've been more than happy to ski 3 feet of BC powder on the Others (78mm waist, remember), so the Mantras seem like they're surfing.
On the Mantras, I can do what I want. I can lay them over on corduroy and they will carve a reasonably sized arc without chatter. In the trees or bumps, I can suck my feet up and pivot steer the skis easily, when necessary. They tolerate my sloppiness, which happens too often.
With all of that said, I probably wouldn't own the Mantras or any other "big" powder-oriented ski if I still skied mostly at MJ. The Mantras are not well suited to the narrow stance I adopt in the bumps, and I can easily ski Colorado's light powder on the Others. And the Others give me more face shots, since they run deeper. Also, at a big ski area, the powder gets skied out in a hurry. Even the "secret" stashes.
So, there's the monkey wrench. Unless you're in BC or Utah, how often do you really get to use a pair of dedicated powder boards?
On the chatter issue: Chatter can certainly be aggravated by tune. I believe, however, that technique issues are often the root cause. Attempting to push on your stance ski, rather than simply balancing accurately is one thing that can easily lead to chatter on very firm surfaces. And just because one ski doesn't chatter and another does doesn't mean it's entirely the fault of the ski or the tune. Some skis will have more tendency to chatter on hard, scraped-off snow. When they do, though, you can often make it go away by adjusting balance, edging, etc.
Of course, all this is opinion, and Your Mileage May Vary.
My posts strongly suggest I have the intelligence of your average kitchen appliance and the personality of a cinder block.