You can learn more playing third. Listen to the harmonies and the chord changes. You will be a better lead player if you have a solid understanding of the other parts.
Also, you will generally be able to contribute more to the improvement of the overall sound from third. Bringing out the harmonies by playing a solid third part will flesh out the sound of the band. Don't let your ego get in the way.
Funny thing, many commercial big band gigs I play, people fight over the third book. Same pay, less work.
