Keith L.-
Honestly, I've used both, and I don't really have a preference. You can get more fluid in a 10.50, but 8.50s result in a narrower tractor if you've got a 36" thrower. Of course, all I use for snow removal is 82 series tractors, and they look dumb with 8.50s on.
On a side note, I'm wearing shorts here in sunny Florida, where it's 83 degrees, but tomorrow night, I'll be back in IL and the cold...