Hi R.,
I am traveling in about 2 weeks by plane, and it will be the first time for my 9.5 month old daughter. I got her a seat using my airmiles (look into this if expense is a factor) and I plan to borrow a baby b'air from a friend for when she needs to be on my lap.
She is at that stage where she wants to move around a lot, and turbulence can be unpredictable. I don't want her to become a projectile object, as another responder mentioned. So I feel much more safe with her in a seat, but I also thought that with the baby b'air harness, I can feed her and play with her safely. She also does not like to sleep in my arms, but is comfortable in her own seat.
Also, I am not sure about your son, but my daughter is a creature of habit, so I made our reservations with her feeding and naptime in mind. Of course, her schedule may change a bit, (though it hasn't so far!!) and the flight could be delayed, but I booked a time where I could feed her and then she would be ready for a nap once we take off.
Other than that, with him in a seat, you can have both hands free to entertain him, and if there is turbulence, he can be extremely safe from the uncomfortable motion.
One last point: I usually use a snap and go, but instead I am going to bring her regular stroller - I can put the carseat in it as we walk through the airport, with the carseat base in the bottom basket. I will gatecheck the stroller so it is available to me as soon as we deplane, and then I have her stroller where she can sit up to see or be fed, if need be, and I can also recline it for her to sleep if we are out and about. We still have not decided whether or not we are renting a car, but if we do, or even if we take a taxi or shuttle, I don't have to worry about her being unsafe in a vehicle.
I hope this helps. Whatever you choose, have a great time!
Good luck!!