MT Buller or Thredbo

very good question, i have done thredbo myself (dh) and can vouch its incredibly good fun, although sadly its only 1 track on the whole mountain open to riding, it is however nice and varied, some good tech, very fast and flowy in parts (unlike many other nsw dh's) some truly retard corner designs in part! and lacking in air time if thats your bag, although i hear raw nrg (the crew previously taking care of trails/bikes hire etc) has now changed and this year might be a bit different..all up the thing has chairlifts and its 10 mins to the bottom for average riders like me, so yes its worth it for sure!
as for buller, i want to go, i hear its really come along well, and has 2 different style trails to the bottom and is top fun and gives thredbo a good run for the money now..further than thredbo by quite a bit, thats about all i know personally..but yeh, thredbo is tops, best ride ive done in nsw personally (although if mt coramba had chairlifts it wouldnt be far behind!) and driving down you can hit some canberra stuff on the way too..