I have done both ->
My 1 yo cat HATES the car... she cries until she wears herself out.
However, she rather enjoyed the plane... as odd as it sounds, the noise seemed to calm her. She meowed a bit during take off, but she slept through the entire flight AND touch down. She also seemed to enjoy the extra attention from all the other passengers (never out of the carrier) and tuna treats.
She was able to "hold it" for the entire time, but I just wrapped the bottom of her carrier with a "diaper pad" (absorbant sheet... available at pet stores) and blankets (washable... just in case).
Family members of mine had a similar experience with a VERY hyper puppy... the plane had a calming affect and put him to sleep.
Either one I'm sure stresses the cat out, but it definitely sounds like you'll do whatever you can to make your little guy comfortable whether in the car or plane