he actually shouldn't be letting you pick them anyway. a good breeder will tell you which pup is the best fit for your plans and activity level. If there are a couple pups that match your needs, the breeder would then let you choose between.
Someone meeting the pups for a few minutes aren't going to have any inclining of their personality. The pup that is happy snuggling in your lap could be the wild child who spent the last hour terrorizing the household. The pup that is running rampant and doesn't want to sit still for attention may have just woken up from a nap and is usually the snuggler of the bunch