I think it is important to remember that pups with the breeder that little bit longer will have experienced less, especially if they are kennelled 
Once again Rachel, you are generalising.

Maybe I'm the exception rather than the rule but when I had Neneh's last litter, I had three pups with me (including Halle) up to the age of 17 weeks.
One was staying with me as the new owner's were an enquiry that came after 8 weeks, and they had a holiday booked, and didn't feel it was fair to have him for a few weeks and then bring him back. The other was here as a result of time wasters, and awaiting a suitable home.
It was damned hard work I'll admit, but every day those pups went out individually, at first being carried and then later down for short walks which included a small town's high street. We even took all three out a couple of times for an off lead run. They all turned out happy, sociable dogs who love the car, and the boys took no longer to settle into their new homes than their siblings.
It could have been very easy for me to have left the two boys at home and just took Halle out as she was mine, but as a responsible breeder I wanted the best for
all the pups.
Don't put all your eggs in the same basket, and don't put all breeders in the same kennel!