Build the Helldrakes so you can swap between flamer and autocannon..its really no worse than an IG Hell hound..only slightly harder to kill.. (20 orks firing at the back side drops it like a stone) or 9 Lootas on the front armour..)
Yeah 20 Orks in the rear drop a Helldrake like a stone if and only if by "drops it like a stone" you mean on average does less than 1
HP of damage (0.74
HPs to be exact, meaning on average you'd need 80 boys in its rear arc to kill it). Likewise 9 lootas do on average 0.67
HPs to it in the front arc. I can only presume you play your Helldrakes as starting with just 1
HP? In which case I suggest you reread their rules and you'll be delighted to find out they have 3.
As for the Hades on it, whilst I love the Hades as a gun Bs3 is too unreliable. For 170 points getting just 2 S8 hits is not a great investment. The flamer cares not for the Bs3 so should always be taken on your first 2 Drakes only then should you consider the Hades.