If the roofer is good it will not matter what brand shingle he is using. As mentioned on the other post, roof shingle warranties are pretyy much worth less than the paper they are printed on. He may buy the GAF better or get a rebate but if the GAF work better for you, get them. If you feel comfortable with the roofer, use him. The shingles get installed the same way regardless of brand.

Agree - warranties are marketing. Install what you like from a reputable brand that is similar in it's marketing warranty - i.e. 30 years, 40 years, etc. so it's over similar quality (good / better / best kind of thing)

Pay more attention to the underlayment, flashing, decking, ridge vent, and all the "invisible" stuff, than you do the shingles. Those are the questions to the contractor that make the biggest difference in wear, and avoiding leaks.

+1. Certifications are just part of the calculus you should use to evaluate your roofer.