I was running my xl5 rusty in the snow tonight and all of a sudden the whole thing just stops in the middle of a run. I could tell by the sound it made that it was a gear mesh problem. I took it inside and when I took it apart I noticed there were teeth that had broken off of the spur gear. Luckily the pinion gear looks fine. I plan on getting the same gear (48p 86t) but am wondering if there is any better brands or brands I should look out for?
Well people have already answered and told you plastic gets brittle in the snow but I personally don't think that was the full cause of your problem. The cold and brittle plastic might have been part of it but I also think your gear mesh might have been loose also. I doubt the brushed motor had enough torque to bust some teeth off the spur even if the plastic was brittle from the cold. Check your gear mesh often so you don't need to buy any more spur gears.
As for what brands are better and what you should get, just stick with the stock traxxas spurs. There cheap and easy to get ahold of. When you buy your new spur buy one or two extra it's always good to have a spare on hand. Steel spurs are made by a few company's for traxxas transmissions but with the brushed xl5 setup you currently have the steel gears are overkill. You don't wanna buy a steel spur just to fix a problem that can be solved with proper gear mesh. Also having the plastic spur break is a lot easier to repair then having a gear inside the transmission break. A steel spur won't break but it will transfer the force to a gear inside and break that gear instead. You always wanna provide your car with break point that is the easiest and cheapest to fix.
Oh and make sure you dry your car off after running in the snow! If you can blow some compressed air threw the motor also to get all the moisture out.