Especially since it is a supported tour, I see no reason to keep it short. So do the one that makes you happy.
My first tour was self supported, 73 days, and 4244 miles. It was the first tour for my two companions as well. None of us regretted it.
I can see some merit in doing an overnighter to get gear choices sorted out for self supported touring, but definitely do not think it is necessary for experienced campers or folks staying in motels and eating in restaurants. For fully supported tours I see way less benefit to the short shakedown tour.