Actually Ambien lasts longer than Sonata....Sonata is very short acting. Puts you to sleep but might not keep you sleeping.
Ambien should be preferable to benadryl, because you will have more of a "hangover" with the benadryl and Ambien should be fine for a 4 - 5 hour sleep. You should wake up without trouble.

Also for people going long distances, you might want to ask about Provigil. It is a new drug, actually developed for narcolepsy but being used by the military for fighter pilots etc. to fight jet lag without lack of performance. It keeps you awake, but not "speedy". And when you want to fall asleep, you fall asleep. My husband (a physician) and I tried it with our trip last month to Asia (12 hour difference) and we didn't miss a beat. We took Ambien on the flight, Provigil when we got there and didn't miss a beat from jet lag. It was amazing.