I have skied both in 177 cm.
The waist of the Thunder was 89mm; next year's Mantra was labeled as 96mm. The Thunder was mounted with a Marker alpine binding on demo tracks; the Mantra had next year's Marker alpine-touring binding.
I am 5'11", 165-170 lbs, 55 yrs old, PSIA L3. Venue was Whitewater, British Columbia. Conditions were soft, with some powder/crud 2 feet deep in places, heavy, no prime blower powder. Conditions included some frozen chunks underneath. Skied trees, bumps and groom, with a small bowl thrown in. I do not huck cliffs.
These are both excellent skis, with the Thunder being the better speed machine and the Mantra being more forgiving. Surprisingly, even the Mantra will carve well on corduroy, although it probably sucks on really firm snow. It's easier to get a pure carve out of the Thunder, though. The Thunder is also stable at high speed on firmer snow; it settles down and runs. The Mantra can be piloted to short, round turns more easily than the Thunder. Neither ski requires active unweighting, at least on firm snow; they can both be driven smoothly from one turn to the next while maintaining good snow pressure. The Thunder, in particular, might be stiff enough so that some will believe they have to unweight it, but it responds perfectly well to a smooth roll from edge to edge.
Both skis were also perfectly willing to pivot-slip. Neither is particularly well suited to bumps, but on the large, soft bumps at Whitewater, they both did OK, allowing easy smearing down a zipper line or larger arcs as desired. Use plenty of A/E in the bumps, as always.
The combination of tight trees and deep snow was more easily handled with the Mantra, which floats better (not that the Thunder tended to dive - it didn't) and allows easy foot steering when space is limited. The Thunder took a bit more effort and focus, but could be skied in the same lines.
Both were delightful in the bowl; the Thunder preferred a bit more speed with somewhat longer turns.
The width of the Mantra kept it above some of the refrozen junk that was underneath in spots somewhat better, and it had less tendency to get stuck in a groove with a frozen wall of coral on each side.
Both skis reward power and accuracy, but the Mantra doesn't punish you as much if you're a little off.
The graphics on the Thunder are awesome. Put the skis together to get the full effect.