go with the XXX-S. Its more consitant and its so smooth. I also like the support the Losi Team gives you if you ahve any questions. All you have to do is e-mail one of them and u get a reply withen 24 hours wich I love. The XXX-S in my expierence is an awsome car and is very user friendly and easy to change parts and anything else you have to do. Oh did I say that its a very fast car. I've also noticed that the XXX-S has won more races on the Pro level than the yokomo. If you get a XXX-S more people have that car than the yokomoe where I race. So you would have more help if more people have one car than the other. If you want you can try both cars. Test one car and then test the other one.
I hope this helps you.