TL;DR? I wouldn't upgrade my 5.7C for the Mach 6, but if I was in the market for a new bike, there would be some serious test rides happening and the Mach 6 would be a serious contender!
The longer version
I'm in love with my 5.7C
I wouldn't say it's an XC bike (having come from a Mach 4 previously), but the stock build comes on the Trail or lighter side of AM. That said, it could certainly be built a little bit slacker and burlier if you so desired, which may be my next step once things start to wear out
It can also take on a lighter/steeper edge for a burly marathon bike too
Mine is currently in 1x10 guise, with a Revelation WC @ 150mm (for approx 67 HA), Fox RP23 (thankfully not CTD), Reverb Dropper, FlowEXs on Hopes with a Hans Dampf 2.35 up front and a Specialized Purgatory 2.3 out the back. This weighs in at 12kg and sees me through everything I can throw at it around Sydney and beyond, including the climbs
I've also got a set of Crests on DT240s running a NN and RR, which when installed on the bike with the double-ring chainset, take the bike to 11.4kg. This is fine for most trails, but the slightly heavier build is less worry and give me more confidence to really bomb rough trails at speed.
As much as I like the idea of the Mach 6, I'll be keeping my 5.7C for a while. The extra 10mm doesn't seem like a justifiable upgrade unless my frame breaks (and I cannot see that happening anytime soon). Sometime in the near future the shock will be getting an upgrade, as the RP23 is starting to show signs of lackluster performance and I'd rather put the servicing costs towards a KirK. I'll most likely change over the fork to a Pike at that point too. That would, by my (most likely highly inaccurate) arm-chair calculations, put the bike @ 12.5kg, with a 66 HA. Pretty burly, quite aggressive, but still nice enough to pedal just about anywhere! It will wander a little bit more on the climbs, no doubt, but I'm sure I'll adjust.
And lastly, I'm not entirely sold on the aesthetics of the Mach 6. One of the things I love so much about the 5.7C are the clean, crisp lines (on my medium frame, anyway). I'm not 100% sure I like the way the top-tube joins the seat-tube (with that little gusset). I'm sure it will ride like a demon though, and once you're on the bike you never see it anyway!
I hope that helps! Let me know if you need more info and I'll see what I can dig from my memory for you