A lot of contractors go with personal preference… That being said, both companies offer superb warranties! Generally I do not stick to a particular shingle, I asked the homeowner what it is exactly they are looking to accomplish. What I mean by this is are you going to be staying in the home, are you selling, is this a rental, etc.? Price is also usually a factor… To answer your question, they are of equal quality! Just make sure that the contractor you hire is reputable and takes the proper steps in nailing! 
Certainteed are the most sold shingles in the area in which I live in . I think they have a better color selection . Now GAF are the worlds leading seller on shingles. I have personally installed a lot of both & I’d have to say both brands are 2 of the best for quality & looks . It’s not always the shingle you buy but the installer that installing the shingles.
When purchasing shingles, look for the weight of the shingle. We want ot purchase the greatest amount of asphalt for the dollar, trust a shingle manufacturer who stands by their product, and will be responsive if a problem arises.
In our area, Certainteed is the leader in all of these areas.
I have been in the roofing business since I was 18. I have worked with Certainteed and GAF, both a good quality shingles. certainteed is a little heavier shingles and I personally like their colors more. A roofing system is only as good as the installer that installs it. Roofing shingles is only a part of the roofing process. Wish you the best.
Unless requested by our customer, we always use CertainTeed.
most prefer
shingles
There both excellent shingles…best answere is what company swiftly honors their warranty and answers technical questions . In my 16 years experience that has been GAF .
They are both great shingles. I generally sell Certainteed as many of the products I prefer are Certainteed and I can direct a customer to one website for shingles, siding handrail and many other products.
Here in the Northeast - Hurricane prone areas and cold weather areas, I prefer to use GAF. They have a splendid selection of colors suited for NY, as well as a hurricane-proof shingle able to withstand up to 150 mph hurricane-force winds. Backing that up is a LifeTime warrany of 45 years on the Timberline HD shingles - so I go with them.
In comparison with TImberline, the pricing of the HD Shingles tends to be more affordable for folks in NY.
We also like to install the GAF Cool Roofing System, which involves the use of soffits, ridgevents, ice & water shield, and GAF Deck Armor. That way, no matter how cold and wet our seasons are, our residents can be sure their roofs will work well. Not only are they energy efficient, and green-building approved, but they also have a mildew-resitant coating and UV protection.
I feel that even though Certainteed produces a good architectural shingle now GAF is a better company when it comes to warranty issues. At the end of the day shingles are really just a commodity made up of just a few raw materials. The way a company handles problems when the do arise is what I look for in a building material.
Today we only use Certainteed! I can give you a couple of major reasons that matter when it comes to comparing with GAF. As a product Certainteed standard laminate shingles weigh 33% more than GAF Timberline. So where did your money go when buying the Timberline product? Well I have asked several GAF representatives. I was able to do so easily by being a certified Master Elite contractor for GAF for three years. Going to their wealth builder conferences I meet some people for GAF that should have been able to give a suitable answer. Needless to say I never got one. They all keep throwing ASTM standards in my face. I guess real world answers were to hard to come by. So, the second major reason is granule retention. As far as both shingles go, after they have been installed for a few months and completely cured, they have similar retention characteristics. Now the major difference is during install. Certainteed surpasses GAF by a landslide. I do not have the technical expertise, but I would assume it is due to GAF taking a litteral 60 pounds of asphalt out of their product per square. So why is it important during install? Well, it makes for a shingle that will be less likely to fail due to its only solar protecting feature, GRANULES staying intact! By staying imbedded in the asphalt it was manufactured with, and not falling off during install. This is a really big issue during the warmer months. As for warranties some say GAF handle them the best. Well from my personal experience they do not. I have yet to have any with Certainteed, but They are known to handle warranties and lawsuits well. They do not run tail tucked and file bankruptcy like the other one. I can go into other reasons, but I feel these are the most important. May you all thrive and do your best. Hope this finds you well.
Gaf