HI there, and welcome. All my dogs have been older rescues but I have read around this whole subject just because I am interested in dogs and their behaviour anyway - this is some of the things I read:
There is a huge area of debate about which puppy you should actually pick. Some say you should pick the one that first comes to you. Some say avoid the one that is first on the mother's milk bar [this is likely to be the most dominant one] others say you should remove all the puppies from the mother and see which one the mother picks up and carries back to the 'nest' first..... Some say you should put the puppy in your arms and hold it on it's back and see what it does, does it try to get up? or does it just lie there? does it put up a fight? for how long? they advise against choosing one that doesn't submit and one that just lies there with no resistance. Avoid the smallest/weakest one - much as it may pull on your heart strings to want to take the weakest one - they may [not saying they will!] have health/behaviour problems later on.
Others say you take a squeaky toy or ball and roll it in the den and see how the puppy reacts - what does it do? is it frightened, curious, for how long? Avoid one that keeps well away from it and go for the one that is a little hestitant at first but then has the confidence to explore it. Middle of the road type pup I guess. There are no end of suggestions like this, these are just a few that I remeber reading over time.
Not sure, I hold that much sway with any of them, tbh. I think at this age, it's not that possible to judge how the pup will mature, especially as environmental and herditary factors will all play a huge part in shaping your pup's eventual behaviour/attitude too.
Far more important that you check out the temperment of the parents of the puppy and that you get satisfactory answers about the health and heart testing of the parents too.
Here's a link - you might find the latter part of it interesting
How To Choose The Right Puppy For Your Family
Hope some of this helps anway, keep us posted.........we just LOVE looking at puppy pictures here!