As far as brands go, many snowmobilers are fiercy brand loyal. It is kind of like the Chevy/Ford/Dodge thing. Which one is better? Beauty is in the eye of the beholder as they say.
I like my Ski Doo better than Polaris/Arctic Cat/Yamaha.
I like my Chevy better than Ford/Dodge.
IMO, you are more than likely to get a decent sled from any of the 4 manufacturers. A lot depends on how well the previous owner took care of it.
Try this link for a well written used snowmobile evaluation guide...
http://users.metro2000.net/~rmmc/usedguide.html