weight is the primary difference. when you remove the counterweights and weigh them, the 12.6 is lighter than the 12.5. you can select either standard copper or Valhalla wiring for either 12.5 or 12.6 arms.
whether you get the 12.5 or 12.6 (or other JMW arms for that matter) be sure to get the new version that has fine VTF adjustment via a screw that is machined/threaded into the back of the arm wand. this allows fine cartridge weight adjustments without offsetting the azymuth. OH YES, FINALLY!
the new support armbase and dogleg set screw/post coupling assembly is better at keeping the cartridge aligned when adjusting the VTA. with the previous version you can see the cartridge/wand move as you tighten VTA adjustment knob when coupling the vertical support bar (in front of the cueing lever) to the plinth.
as to the tick tocking... the 12.6 arms cannot be damped because you must use the 12.6 bearing assembly, which does not allow damping. this will require that you use a good tt stand/base and flat lps or a ring clamp to minimize wobbling about the unipivot. if i recall the 12.6 was designed for lower compliance (or is it lower weight? please check w/ vpi on this) cartridges that do not need dampening. you can use the 12.5 arm wands with the 12.6 bearing assembly. You cannot, however, use the 12.6 on a 12.5 bearing setup.
