Adopting from shelter verses rescue

I look into adopting from a golden retriever rescue, adoption fees for dogs between one year to eight years old is $250 dollars. And then while browsing the classifieds I find a five month old golden retriever mix at a shelter. His five months old and the shelter would only charge about $85. At the rescue website it would have cost $300 for a dog that young. So I have to ask, whats the catch?
   
The shelter claimed to nueter/spray and give heartworm medicine just like the rescue agency. Is there an actually difference between adopting a dog from a 'shelter' verses a 'rescue' organization.
   
When I first started looking, I had the impression that rescue organizations give the dogs more vet attention and make sure they are healthy while shelters didn't necessarily keep a close eye on the animals health. Was I mistaken?

I've adopted from both shelters and rescues. There are a couple of differences:
   
1. Rescues are not gov't funded, shelters usually are, which is why there is such a huge adoption fee difference.
   
2. Rescues sometimes specialize in one particular breed, shelters usually have a mix (but you can find lots of purebreds in them).
   
3. Rescues tend to have more time to devote to the animal, either by the workers themselves, or by foster families, that help to learn the dog's behavior and work out any potential problems. They also learn the dog's personality and can better match up a person or family with an appropriate dog. Shelters are usually spread too thin amongst all the different animals they deal with on a daily basis to give undivided attention to any of them.
   
4. Rescues are usually no-kill. Many shelters are no-kill as well, but there are more that do put down after a while.
   
I don't want you to think I'm steering you towards a rescue since there are many good and bad rescues AND shelters. Do your homework on whomever it is you decide to adopt from. Ask around, ask for references. I do implore that you look for a no-kill shelter, though. Then you're saving 2 lives....the dog you adopt, and the next dog who can come in and take that space. :)

The main difference seems to be that the rescue people try to give you an indication of the pet's personality & if they get along with other dogs/cats/children, while the animal shelters just know that it's not likely to go nuts & try to bite you.
   
You generally find more "problem" dogs in the rescue groups, as they have the people to work more individually with a dog's issues.