It was a Charge Ti Spoon until I actually measured my sit bones and confirmed something that I've known in my heart-of-hearts for a while: I have a wide arse. Bought a Specialized Henge Comp 155mm and it's bliss!
According to an interview with Dan Stanton, the only real difference is that 853 is more resistance to dings than 631, but at the thicknesses of MTB tubing that difference is basically null and void.
Love Stanton's bikes - just a front and rear triangle with nice geometry. The 29er looks great in the larger size and the finish on that frame is lush.
Being a shortarse, I just took delivery of a second-hand Slackline 631 and I can't wait to take her for a spin.