This is a very good topic and you have gotten a lot of good feed back so far.
We have had anywhere from 1-4 dogs at a time. And based on their personalities, I could tell you which ones needed a dog friend and which ones would have been perfectly happy being the only dog in the house.
I would say for the most peace in your household...get a second dog if your current dog loves both dogs and people. If your current dog is an "alpha" type...then pick a dog friend that is the submissive type. You don't want a "top of the dog chain" power struggle. We have had that situation a few times and it was anything but fun.
We currently have a passive male sharpei (4 1/2), an "alpha" type male bulldog (3 yr), and we just took in a 9 month old lab/hound mix female that a family member was not able to keep. She is cute & full of puppy energy, bad manners, a loud barker & a he** of a lot of work (we started training classes which is helping). She eggs on the two boys to play with her which they don't want to do...then out of no where the bulldog will pounce on the sharpei because he thinks he is taking away some of his attention from the "new girl."
So...whatever you do...please really put a lot of thought and time into the decision...because you want getting the dog to be a good thing for you guys and your current pet. The saddest thing is when people get rid of an animal because things did not turn out as they planned. It is a lot of work...but you get a lot of positives in return.
(p.s. do these 2 dogs look like they want to play with a puppy...ummm...no...well...she is staying so they will have to find a way to all get along).
GOOD LUCK!