I cut the post on my m's and angled them forward to get farther forward, jumped on a pro and made it my b1tch right away. (that should tell you it handles great) the biggest difference to me was I could feel the slight angle of the handle bars on the pro and did not feel as balanced on the pro, but the pro's ability to correct itself made up for it, the m8 feels more grounded.
Biggest handling difference I found was the pro, its light on the skis and uses a lot more counter steering than the m's do. One thing a pro does a m8 just wont do is down hill turns. On the pro you just lean and it cuts to one side very easy, on the m8 you have to step back or get weight on the back of the tunnel and help the a$$ end drop down, big difference. Even though I have cats that weigh as much or less then a pro the pro feels lighter and easier. I can tell the power isn't there on the pro but you can still make it work.
The pro is narrow too allowing it to hang on a sidehill with little drag, the m8 drags sooner than the pro because of its wide running boards and body, but the pro running board are to narrow and will kick your a$$ when the snow piles on them, they need more grip. The pro puts the track down farther forward than the m8 and thats part of the why it responds to rider input well and light on the skis, the pro 155 track puts down nearly as much track on the ground as a m8 162 and is shorter with a closer distance from ski to track, it balances out and responds very well.
I put a kmod suspension under my m8 which gave me another 1" of suspension and changed some other stuff making it so I could hold any line a pro could and maybe more, I like the m8 better but it does take more effort, seeing how I am 6'6" and 210lbs its not an issue but for someone under 6' or 180lbs I don't know if they would be able to make them do the same stuff. Who knows what a pro would do with the same suspension under it though.
I hate the pro motor though and everyone knows it, get a warranty at the very least, or a cat. I think you would pick it up real quick on the pro rather than the m8 unless its modded, but it will be interesting to see what the new cat does.
Where are you going to be riding