You know, when it comes down to it, sled choice ends up being a personal choice. Other peeps are going to have their own biased opinions. I will NEVER purchase a Yammie, they make a decent thumper motor but aren't trying very hard to reduce weight. I will never buy an M8 because the ergonomics don't fit me. If I were to buy a different sled it would be a Poo, but the D8s are blowing up left and right and I hate the running boards.
I happen to love the way XP fits me, the power to weight ratio and the way the thing handles. Yes, they have their issues but what sled doesn't. My 08 XP was a flawless sled but I replaced the leakey oil tank and an air bubble in the oil line caused my sled to squeek a piston. I got that replaced and she was running great right up until I hit a hidden rock at speeds that would crumple any sled. I got an 09 XP and I love it, just gotta clutch it a little more. If you take care of your 2 stroke you can ride it for years. I have seen many Summit 670s out on the snow this year, My buddy has an 05 Vertical Escape that still runs strong and I know of one guy that has an 05 M8 that has nearly 7000 miles. Its all in how you take care of the sled.
As for an M8 being a better sled, I don't believe it. I have seen quite a few M8s and M1000s attached to the wrong end of a tow rope this season, just as I have seen XPs or Dragons. They are pretty much equal in performance. Riding ability is 90% rider and 10% sled.
The only thing I wish Doo had was fuel injection. My next sled will probably be the 1200 4 stroke when it comes out in a mountain chassis. The stock motor is stronger than the Yammie and when you throw boost at it, it is AWESOME! I saw a 1200 that was converted to a mountain sled this season and it was boosted. That thing was the most unreal sled I have ever seen, it was light, powerful and handled just like my XP, but with 150 more horsepower.