I'd agree with ME...unless you feel really comfortable with the breed as far as evaluating goes (and what will work for you on a case by case basis), I'd say go with rescue. With a shelter, they in most cases, if they do accept this breed, have to accept ALL dogs of the breed, including the unsound ones. Most responsible shelters will put down unsound dogs, but I can't say it's rare that they keep moderately unsound dogs up for adoption either because they want to try and spare them or because they don't know any better.
With a RESPONSIBLE rescue, you will have a good picture of the temperament of the dog (a lot of times dogs act differently in shetlers than in a home environment), how he is with other dogs, cats, kids, birds, and caterpillers. You are less likely to come across a dog with a health problem unless they tell you up front about it, and less likely to end up with a sick dog (a lot of times shelter dogs come down with kennel cough shortly after leaving the shelter). Plus, you'll know that in most cases, they specifically picked that particular dog out of best case ten dogs, most of the time way more, available for them to choose from...it's much less of a "crapshoot" so to speak.
Honestly, I'm not a huge fan of shelters adopting pit bulls out directly...it takes a lot of work and effort and time to do it appropriately and truth be told, most shelters just don't have those resources available. I'd always say with this breed, go to a rescue.
To find a good rescue program, start at pbrc.net or petfinder.com (here you'll have to search for "pit bull terriers" and then go from there). Inquire on the ones that look good to you and just see which rescue seems to best fit your requirements, and go from there.