Cargo V. Cabin all depends on the airline. For instance, BA wont let pets in the cabin but Delta will. Most airlines will allow pet travel in the pressurised (sp?) cargo hold however only a select few will allow international travel in the cabin: These airlines include, as previously mentioned, Delta, Continental, KLM, Northwest, and Canada Air. There are others but you should check direct with the airline.
Under both circumstances a fee is charges as the pet is considered extra baggage. For the cargo hold there is generally a standard pet carriage charge as well as an extra baggage charge. For the cabin it's (as far as Im aware, for Delta at least) a standard pet carriage charge. About $75 per pet.
Only two pets are allowed in the cabin per flight so you should book your pets in when you make your reservations. Pets traveling in the cabin must fit in a carrier underneath the seat. You can get carrier specifications from the airlines but the general rules are that the animal must be able to fit comfortably inside the carrier and be able to stand and turn around while inside it. I'm travling with two cats in cabin in the next couple of months and I have chosen Sherpa brand bags which are readily avalible in US pet shops like PetSmart. They are approved on certain airlines for cabin travel only. You may not use sherpa or any soft sided carriers for cargo travel.
Cargo travel must take place in a solid sided carrier that does not run the risk of popping open or breaking during flight. Normal plastic carriers are not generally reccomended. You can have wooden crates made for your pet for cargo travel. Carriers should not have wheels or the wheels should be taken off prior to releasing the carrier.
Some flights will not allow cargo travel for health reasons - for instance if youre flying in the dead of summer to a hot location your pet may not be able to fly on day flights as the pet can sometimes be kept waiting on the tarmac in preperation of off-loading.
In both cases pets that show any signs of being sedated will not be allowed to fly.
So basically you choose whichever suits your circumstances best. Large animals cant fly in cabin and must go via cargo. Small pets can travel either way. As I said earlier, I have two cats that I'll be taking in the cabin with me. I've chosen this route because I have nervous cats, one of whom is quite old. I would like to keep a close eye on them in flight. Its also to aid in peace of mind. I would be worried for 10 hours straight if they were ok in the cargo hold because thats the kind of person I am. Ive heard lots of good stories about people taking their pets by cargo but I myself would rather have my "kids" close.
Hope that helps,
Sarah
