I rode a 600 for years in the mountains. With that said, an 800 is definitely the best way to go. It's all about horsepower to weight ratio. And considering that the 600 actually weighs 4lbs more than the 800, well....
At elevation, a sled loses 1/4 to 1/3 it's horsepower. When I ran a 600, I dug myself out a lot. With my 800, I shake it out or roll it out. Rarely ever need to dig. A 600 doesn't have the snort to shake out. So, an 800 isn't gonna give me a heart attack.
You could lighten a 600 with reduced strength parts, but then the reliability you want would suffer. And I'd say the new 800s are incredibly reliable. It's also impossible to sell a 600. Not sure why they even make 600 mountain sleds anymore....
All very true, but I have a hard time believing 600s are harder to sell when I look through the swapmeet and it seems like a majority of the sold sleds are the 600s. I know my family has bought 2 off this site to say the least