Depends on the role it shall perform for:
- as specialist railguns work well
- for piracy without hatchbreakers small beams work wonders
- backup weapon i'd stick with bursts (only if you are an up-close fighter though)
- intimidation and hull-finishing: El Torpedo!
If you just want a decision between MCs or Cannons go MC,
they perform better and also look better.
I would like to suggest beam turrets for taking care of all those annoying eagles and vipers.
When engineers comes out, I think it will be far more interesting as you will have a variety of weapon special effects to consider.
I prefer the multis on my 'vette. The PA's strip shields well enough so I put multis in my other slots both to conserve energy for the PAs and because they do more to hull than lasers.
The ventral mounted C3 is a beam laser, just in case I need some shield cracking power but can't bring the PA's to bear.
It really does depend on what you have elsewhere. I ran with C2 cannons for a while, so C1 cannons to add in made sense. I like multis in general, but in a Corvette, the kind of ship they'll do something too are the kind of ships your other hardpoints will melt anyway.
With 2.1 coming soon, I'd vote for missiles or torpedoes.
Put torpedos in your small hardpoints. Trust me. You don't lose much DPS, but you gain a nasty surprise for any other CMDRs you come across who decides that they want to mess with you. Drop their shields, launch torps, watch huge chunks of their hull disappear.