Boysun or Nurata mountains better?

To my mind the scenery is slighly more interesting in the Boysun area, but it lacks any sort of tourist infrastructure, so unless you are fluent in Russian I'd head to Nuratau, where there's an excellent organized homestay program (see website listed in LP). Also, if you go to Nurata in the winter you're almost sure to be able to catch a Buzkashi (Kupkari) game.

Boysun easy enough to get to but you'll have to make a few transfers. From Shakhrisabz take a shared taxi to Guzor, then another shared taxi to Dehanabad (I think). From there you should be able to find rides to the Boysun junction (you may have to get in a Termiz-bound taxi or van and have them drop you off). At the junction (near Darbent) there should be shared taxis waiting to take people to Boysun. You may get lucky and find a shared taxi directly to Boysun from Guzor or Dehanabad. From Samarkand the process is similar only you'd start by taking a shared taxi to Karshi. Or you might try taking a morning shared taxi bound for Denau and getting off in Boysun (if they are going that way). From Termiz and Denau there are frequent direct shared taxis to Boysun.