t5's work gr8 with top end accessories i.e. vorce reeds, carbs bigger than 28mm , dune/drag port, etc. etc.
tr6's work gr8 with low to mid power i.e. stock carbs or 28mm or small, stock reeds or rad valves, no port or trail port or what some people call play port.
it really depends on what type of riding you do, and how much you have done to your bike!