Shelter dogs can be ok. If you have the time I would consider getting a puppy if you are not experienced with dogs. It is much easier to train a dog from 9 weeks on than it is to train a mature dog that was rescued. Even if you are looking for a companion pet dog and not a dog that has a job to do there will be a ton of training involved, come here, get down dont get my clothes muddy, go outside, sit down, etc... We rescued a yellow lab off of the road that had been around awhile. I imagine somebody dumped it where we live at in the country. This dog was emaciated and hungry. He had trouble with the oldest dog we had and needed VERY FIRM leadership to get some of that alpha dog out of him. He made a very sweet dog in the end but it would've been easier to get his behavior right from a puppy.
Currently, we have had a mutt of some sort that we rescued from a local vet. He's probably mainly black lab but mixed with some sort of sled dog; short ears and curved tail. He was obviously abused but he is not a mean dog at all. This is by far the most difficult dog that I have ever worked with. He has a great desire to please but he is completely people shy. He has made me learn a totally different approach with disciplining. Even raising my voice at him used to make him timid for up to two days and he wouldn't have anything to do with me because he was scared out of his mind of being beat (and I never beat him). Its taking a LONG time to bolden him up. Again, getting his behavior right from a puppy would've been much easier. You may not want to take on either of those two dogs because they are very trying. Im all for adopting pet dogs but they definitely come with their own challenges.