The 6 was not that popular, it was terrible quality and would break easily. The 7 is a much better car and so the chassis has become a little more popular again. The 7.1 is about to get an update soon apparently.
For club racing the main chassis is Xray, then you have Yokomo and schumacher. The less common ones are the Awesomatix etc. As is always the way its probably best to pop down to the local club and see what they race. There are regional variations on what some of the popular chassis are.
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