Both are good products.
Certainteed helps me promote there product and has many programs for contractors and there free. I recieve lots of support when needed.
Gaf/Elk All I know is I would have to PAY THEM THOUSANDS to promote their product and warranties.
Therefore I chose CertainTeed.
P.S. Both of them and every other manufacture could help us contractors determine an equal ground on pricing to help the whole industry. But I guess they are just keeping that whole coalition to themselves. I think they should offer more support, better programs, better pricing, more rebates to the contractors that are fair, honest, and sell their roofs at a fair price. When i say a fair price I mean fair for the laborers, to the foremans, to the owners, and the customers. lets start at about $350.00 a square for a single layer tear off, one story, no dormers, walkable, etc (easy as it gets). Then adjust accordingly from there. Everyone in this industry is underpaid except for the manufactures.