|I didn't know Baleflamers could hurt Flyers!|
Take the Baleflamer.
Alright, let's get into it.
The Hades Autocannon is undeniably a decent gun with 4 shots at S8, and having it on a Heldrake means you can fire it at enemy Flyers. 12 being the usual maximum AV for Flyers, S8 is pretty darn decent. So why not take the Hades Autocannon? Well, honestly, it's just so damn mediocre on a Heldrake. At BS3, you'll hit 2 shots and against AV12 you'll get one glance/pen. Sure, combined with his Vector Strike you have a reasonable chance at taking down an enemy Flyer, but at 170 points you'd be wanting something better than a reasonable chance.
And the same applies to standard vehicles. You have an OK chance to take out a vehicle over a turn or two.
Then when it comes to firing at enemy infantry, you lack both the volume of fire to hurt hordes and the AP to hurt Marines.
So really taking the Hades Autocannon makes your Heldrake pretty mediocre at just about everything.
The Baleflamer on the other hand is just ridiculous. A S6 AP3 Template that you can place 12" away from the model firing it? Yes please!
Unlike the Hades Autocannon, the Baleflamer has both the capability to hit huge numbers of dudes as well as the AP to deal with Marines and any other 3+ save guys. On top of that, it's wounding them all on 2+. Then take into account that it also ignores cover saves and it's just an incredible weapon.
Need an objective cleared last turn? No problem, just go into Skimmer mode, get as close as you need and Baleflamer those dudes away for the win. Opponent running a horde of Power Armour? Baleflamer fixes that. Are they running a horde of anything at all? Same thing goes. It's also great at clearing heavy weapon teams. Really, it's great at removing any infantry T5 or less that doesn't have a 2+ save, and that's most things you'll be facing.
Then, if you really do need to remove that last Hull Point, you can always Vector Strike, just as a last resort thing.
If you're running 3 Heldrakes, I might consider giving one of them a Hades Autocannon then, but for 1-2 they should all have a Baleflamer.
If you want to remove enemy Flyers, get some Havocs or maybe even give a Forgefiend a spin. There are plenty of better options for it than the Heldrake.