Both types will produce dogs that are a good fit for you, and others that are not. It really comes down to the individual pup's temperament and finding one that is a good match.
One very important thing to consider is the behavior of your current dog. From your post it sounds like a major factor in you getting a second dog is for your current one to have a companion to lend him confidence. That might work. But it also may backfire and you may end up with 2 neurotic, frightened dogs. Puppies are very maleable and will learn a lot from the resident adult dogs both in terms of their outward behavior and in terms of their feelings about things. If your current dog is regularly modeling fearful behavior, the puppy is going to pick up on that and may well start acting the same way.
If you're going to add a second dog, I would recommend seriously considering getting a confident, self assured adult who is already more set in it's personality and behavior rather than a puppy who may end up being raised by your current dog to copy his undesireable behaviors and reactions.