Hey,
In my opinion, the main difference between the two is "Love". Dogs from shelters usually are missing something--that's love. Dogs from shelters usually have been abused or abandoned. And then there's dogs at shelters that have known love, but their owners become allergic or move away and can't take care of them anymore. But 90% of them have been abused or abandoned. It's sad to think about. If you are considering another dog, or a dog, then I would FOR SURE go to a shelter first. Because it's not their fault that they're there. Dogs are like kids, they turn out how their parents have raised them. And in dog's cases, I'm talking about their owners. I've known many dogs from shelters, and all of them have turned out to be great dogs. They just need love and a place of security. Keep in mind that not every dog is perfect, but everyone deserves a 2nd chance. Good Luck!
-Dog Obsessed