This way, you will be close and able to talk to him during the flight. Also, you usually don't have to check in as far in advance as when checking him. I have never been able to confirm the horror stories about forgotten heating in the luggage compartment, but there is a chance, especially during lay-overs, that the animal will be sitting out on the tarmac for some time, which might be a problem with cold or hot weather. I took Zeerox with me in the cabin and would not check a cat if there is any other posibility. Not all airlines allow cats in the cabin.
Some airlines have limits to the size and weight of the animal.
Size and type of carrier. To take the cat in the cabin, the carrier has to fit underneath the seat, so it will be pretty small. Still, I think the advantages of keeping the animal with me at all time outweigh the disadvantages of the smaller carrier size. I used a tough, plastic carrier. Do remember that some airlines have not only limits to the size of the carrier, but also to the weight of the cat itself.