As said above; because there are several tire options and tractors are shipped without rims and tires mounted, you need to choose which ones you want. If I remember correctly, I don't think there is an upcharge for any standard size. I know that the Kioti CK's use a larger tire then the Kubota LX's. But any dealer worth their salt can order you the larger ones for minimum upcharge. (I was comparing the Kubota and Kioti when i bought mine 2 years ago.)
I've got R14's on my CK3510 and they seem to be pretty good in snow. They don't seem to rip up the lawn much, but take that with a grain of salt because my lawn dries out very quickly.
I'd say go check out the new dealer and have him/her give you a quote. There so much more to a dealership then that, but that can give you a pretty good idea on how they are going to be. If they don't give a crap about a potential new customer....
You can also find out if you like 'how they operate', aka are they super formal, relaxed, don't care etc.
Years ago I stopped by a place where the sales guy was so laid back he didn't seem to care less if I bought from him or not. I was early in my search but def was going to buy, I just didn't know if it would be a between a smaller new unit or a larger used one. He didn't even give me a verbal quote. Since then they have lost out on 2 tractors, and a zero turn. Funny thing was, if he had given a crap, I would have bought the used unit that initially attracted me there, right on the spot.