I broke 2 Next SLs, metal bits came loose in the carbon… there’s a revised version which was supposed to fix that but apparently doesn’t. The Sixc is a beefier crank, I’ve seen reports of the same failure but nothing like as often, it seems like they just made the Nexts too light. I thought about the sixc as a replacement but tbh they’re not that light really.
(I actually ended up replacing with a set of old XTR M970s, which are barely heavier than most carbon options, stiff, and very strong… but I suspect you’re after new? I can’t speak for newer XTR)
Crank arm and ring size are both personal taste really. I like 170, gearing depends too much on your riding and your legs but, for whatever it’s worth, I default to 30T. Partly because it’s the smallest that fits on my cranks 🙂