ive been riding DH on my hardtail for the past year (only ridden downhill like 5 times though haha) but still its sooo much fun, you get alot of skill out of it, you get great control over your bike, it might be a bit painful in rockgardens and ruts but if you have the right tyre pressure it can b a blast.
however, if you want to get more specific in your riding, like if you only want to FR and DH sorta thing then a dually will not dissappoint but then again the right hardtail wont either, its really what you want to do? a dually is a much more competitive DH bike though, although theres no stopping you from gettin a sick time on a HT i've beaten a fair few of my friends on their big expensive duallies before, but i cant be hypocritical, i am upgrading to a dually (if i ever sell my bike!!!!!!!! BUY MY BIKE!) because i've kinda grown out of the whole DJ'in scene n street stuff, FR and DH is my calling and sounds like it is yours as well.
not sure how much that will help you, but best of luck with your decision