Originally Posted by upshitscreek
when looking to buy a raft, always get something that can do it's most demanding task.
I tend to go the other way, look for a boat that suits what you will be doing most of the time.
An 18 footer will serve you well for overnighters with that big group, but that is too much boat for day floats on everything but the Colorado for all but 4 weeks out of the year. Determine your most likely type of trip and look for the appropriate boat. Your situation will probably require two boats, one for each purpose.