It all depends on what you are facing.
Against armor 12 the Lascannon is far more effective. Since you are facing a Heldrake, you will probably only get one chance to fire at it with your gun emplacement. The Lascannon has a higher potential to destroy the flier in a single round of shooting than the Quadcannon. It's worth nothing that the Baleflamer is a real menace to Sisters of Battle since it ignores their cover, armor save and wounds on a 2+. That means it can easily wipe out any SoB squad it wants to unless you can destroy it IMMEDIATELY.
Let's look at the math.
Bs4, 3s to hit (you can use faith to make it reroll 1s if you're using your troops to fire the gun, or even make it twin linked if you use Dominions), so 1 hit, Str 9 means you need 3s to glance, 4s to pen. Good chance you will penetrate, and the AP2 gives you a +1 to either blow it up or destroy the real threat, that flamer!
Bs4, 3s to hit twin linked. You will most likely hit with 3 out of 4. At strength 7 you need 5s to Glance, 6s to pen. Since the Heldrake has 3 hullpoints you need to then get three 5s to wreck it or hope for a 6. Without the bonus on the damage table, your Pen won't be as effective either. Since the model can use Daemonic Possession it can also possibly ignore Shaken and Stunned results, meaning your pen may be even more useless unless you get a weapon destroyed or exploded result. So that's either rolling three 5s, or a 6 and another 5 or 6. Most likely you won't and the Baleflamer will kill whatever squad is manning the Quadcannon, giving you little hope of killing the Heldrake in later turns.
Your other best bet is to stay embarked in your transports since the Heldrake isn't as effective against armor as it is against infantry. By staying in your Rhinos you can reduce the chances of your Sisters getting burned alive instantly on a 2+ roll.
Keep your Excorcists behind your Aegis line as well to give them a 4++ cover save. With armor 13 fronts they should be very safe if you put them together so the sides are harder to hit. Make sure the rear armor is against the back of the game board or some impassible terrian to ensure that no Raptors or Assault troops can deepstrike down and use their melta guns to blow them up from your rear flanks.
Good luck friend.