Some pros and cons I can see with both:
Lantern grip pros
- Less tiring to carry big light. This holds even more true on land.
- Still have limited use of hand holding light.
Lantern grip cons
- Difficult to hold light in front of you.
Pistol grip pros
- Easier to point at objects.
- Wrist position is more natural when holding the light out in front.
Pistol grip cons
- A little more tiring to carry.
- You lose the use of that hand.
I have a UK D8 which is pretty heavy for a handheld light. (8 D cells!)
I originally got a lantern grip because I thought that I would at least have partial use of my hand while holding the light.
After a few dives with it, I discovered that I was using the lantern grip as a suboptimal pistol grip, because it was better that way.
I therefore got a pistol grip for the D8 and now am much happier. If you can, try both out before you choose. FYI, I've personally seen more people prefer the pistol grip over the lantern grip.
As an aside, I would really like to try out a canister light, but they're way out of my price range at this moment.
Q: Hey, I'm out to buy my first car with the money I saved flipping burgers. Which is faster, a Civic or a Focus?
A: They're both crap! Get a Ferrarri 360!
Q:
Sorry.. couldn't resist..

(Although to be fair most people did mention the cost factor..)