ar6s have two strips of stiffer (so probably more dense and heavier) wood in the core and thalls only have one, so thalls are softer, probably lighter but I dont think it would be a big difference, aaaaaand I forgot your third question
yeah if the ar6 is even remotly close to the ar5. go with the ar6 cause i love my ar5s i just traded my new ninthwards and went back to my ar5 and im perfectly happy with the ski still.