after reading this guide, it shows that the version 3/4 has the better air flow and the 5/6 has better cooling.

AutoSpeed - Top-Mount Trial - Part One
AutoSpeed - Top-Mount Trial - Part Two

im planning on upgrading from a version 2.
anyone got any advice on witch would be best?
anyone had any experience trying the both?

and if anyone got a cheap one for sale, let me know