I have created a lot of small biters and two turrets with standard (yellow) ammo:
http://imgur.com/aclGFJ5
Gun turret: 477 kills / 9550 damage,
Cannon Alpha: 272 kills / 4216 damage.
Same, but piersing rounds (red ammo):
Gun turret: 559 kills / 8973 damage,
Cannon Alpha: 190 kills / 4712 damage.
So, one standard turret has ~2 times more efficient.
Red ammo, medium biters (it needs a lot of time!):
http://imgur.com/kEQcX4P
Gun turret: 581 kills / 47616 damage,
Cannon Alpha: 482 kills / 43955 damage.
Green (nuclear) bullets, big biters:
Gun turret: 40 kills / 15610 damage,
Cannon Alpha: 26 kills / 10757 damage.
Here the standard turret has the fast same damage deal.
It was by continuously shooting. The starting and folding by Cannon Alpha needs normally a lot of time.