It's intersting that so many of you say go with your gut feeling. That's just what I was going to say. My first cocker was chosen at the age of 5 days. My second was just 4 days old when we first met him. Although I had the pick of the litter with both of them, I made my choice when they were VERY young. The breeders in both cases said that they wouldn't hold me to my choice if I changed my mind when they were nearer to the magical age of 8 weeks (home time!) but I stuck with my first instincts & was very pleased to have done so. As I said, I met my puppies when they were extremely young, then went to visit at least once a week until it was time to bring them home. I think that helped them to settle in at home as well. Neither of them were ever any trouble, it was a delight to be owned by them.
My current cocker was acquired at the age of 6 months (no choice to make), but it was love at first sight.