4x4 is a must for the snow IMO. Tall and skinny tires because the cut thru the snow to make contact with the pavement underneath. Same theory as a 1:1 car. I have never done this bu5 I am wondering how a set of paddles would go in the snow. There are also ski kits for the front. You'd have to doable the 4x4 to use them tho. I had one of this conversions years and years ago... It was fun but no good for a not waterproof esc. Even with precautions take to wrap and deal everything... So there is another consideration... Waterproof everything you run.
If you want to keep the boys running sort of side by side, the slayer is a good option. Of you want to give them something different, go 1/8 scale.
Getting past the winter, what will you, or they be doing with their cars? Do you race at a track? Another cconsideration... Do they run an 1/8 scale track? The 1/8 scale are scary fast if you build it like that, 1/8 scale everything is more expensive. Everything!
I have ALWAYS been a Kyosho fan. I love their kits, I love their design, I love how they drive. That said, not every local hobby shop carries their parts. Traxxas parts are everywhere. If you just shop on line, parts for either brand are no issues, always available.
So if you want them running equally, slayer. (no fighting over who is gonna get to run what)... I hope the considerations pointed out helps to make the right decision.
Good luck!
Please post back... I'd like to know what you chose and why.
Thanks!