I think they are pretty even in most departments. Neither of the chassis are any good for riding on wheels but as a dedicated snowbike the steelies dampen vibration better than a AF conversions. I went CR500 mostly because they may be a little more common and I found a nice one. Some guys argue the KX motor is better due to the powervalve and it may just be. They both have crap forks so either way an upgrade will be mandatory. My recommendation would be to buy the nicest one you can find in either brand because if you go the basket case route, restoration adds up pretty quick. $3K should get you a decent starting point IMO. The seats on both bikes are like a lazy boy. Decomp valve for starting is mandatory although lots of guys don't have them but it just makes the bike so easy to start why would you not. If you can find one and get it set up properly you won't look back unless you are looking for your buddies on their 450s trying to keep up.
M5