This is like a "historical fact" by now. The XXX4 has a much better drivetrain (more solid). But the weight distribution & geometry is suited for more or less flat tracks (US style).
The XX4 OTOH is THE King of off-road, as it works perfectly on ANY surface: Flat or very bumpy! Fast and VERY easy to drive too! The discontinued XX4 is now very, very old (13 years?) - and was designed when motors and batteries were much less powerful. Still OK on low/med grip, but those belts (3) get into serious trouble on high grip (astro).
You may still get it to work today, but it's discontinued and it can be a bit hard to find parts, but far from impossible
And you may need to do some mods in order to make it solid and reliable enough.
Now, a small company called X-Factory tried to combine these two into the perfect car: The X-5. Single belt drivetrain, but with the weight distribution and geometry of the XX4. It was OK on low grip, but we could never get to terms with belt skipping on med/high grip. This car is since a couple of years discontinued (the chassis mould broke). They are now working on it's predecessor: The X-7...
Here's some XX4 (and X-5) upgrades:
http://www.ymr.no/X5tuning.htm
http://www.ymr.no/10ortips.htm
I've seen the XXX4 in action outdoors on several tracks, and it looks fairly good to me! So unless you're aiming at 1st position I'm sure you can do decently with it! Good luck!
