From a German view:

I would never take a dog from an animal shelter and will always go for a legit breeder.

In Germany, before being allowed to take a dog out of animal shelter, you get pre-screened by some people, who visit you at home prior purchase and check your home.
In addition, after anything went fine, you don't possess the dog (like it would be the case for buying the dog at a breeder). It still belongs to the animal shelter and you only sign a contract that you (more or less) lend it from the animal shelter.
Those people could as well take the dog back at any time (does not happen oftenly, but legally it is easily possible).
In addition, the price that the animal shelter charges for such dogs is usually higher (350€+) than average non-purebread dogs bought at breeders.
So already at this point I would never take a dog from (German) animal shelters, and the important issues are not even mentioned yet:

• Why do you want to waive the puppy time, where this is one of the best times with your dog?
• Why do you want to take any risks, that the dog has some mal-behaviour (which was the initial reason for him to be brought into animal shelter)? I know some friends whose dogs are barking violently at all passing cars, one dog tries to kill all huge white dogs instantly after seeing them and similar... why would one willingly decide to take such risks?
  May sound hard, but my live is much too short to deal with unnecessary issues like the mentioned ones...