If I had to chose between the xxx-4 and the xx-4 I have to say xx-4. The suspension on the xx-4 is legendary. This plus the fact it has a built in one way clicker puts it above the xxx-4 in my book. The three belt system is alittle harder to work on than the xxx-4's single belt but with todays batteries and motors it is not an disadvantage performance wise. The xxx-4 is very fast on smooth tracks but is lack luster on less maintained ones due to its lack of suspension downtravel. The xx-4 is at home on smooth hardpack or rutted, blown out tracks. Another option is to get the Xfactory X5 which is a combination the best attributes of the xx-4 and xxx-4. It uses the single belt drivetrain of the xxx-4 with the suspension on the xx-4 giving you the best of both worlds.
The Kyosho ZX-5 is another good choice if you do not mind an asian made 4wd.