I too am a CertainTeed shingle Master installer and yes I will be impartial, if I was going to use the CertainTeed designer luxury style roof personally before I would go with a Belmont shingle over a highland slate just my P.O.. Which is also a high impact-resistant shingle almost 3x the wieght of GAF that you can get a surestart 4 - 50 year warranty with 1500sq ft depending on the amount of current layers you should be around 10k
However you could also do a CertainTeed Landmark roof assuming you only have one or two layers to be stripped from the house now with the same 50-year warranty for approximately $8,000.
Now the GAF shingle you were quoted on is it GAF lifetime warranty shingle also known as T30 . Which carries a manufactures prorated 30-year warranty. Therefore if the GAF roof fails in 5 years they will only one to the materials at a prorated difference currently 15 sq GAF so at current material cost and depreciated value you will only receive about$1,300 towards warranty material and have to pay to have the roof remove dumped.
Now I have a few questions for you.
1- how long do you plan on living in this house when you sell the house do you want to try and utilize the roof and the warranty as an equitable investment tool - if so hands down CertainTeed
2- do either of the proposals speak of 3 foot of ice and water Shield past exterior wall, f 5 or better drip edges, a copper fold-over waist pipe sleeve and flange or forever waste pipe boot.
3-do either cover replacement 16 ounce copper flashing at chimney
4- MOST IMPORTANT have you gone to the CertainTeed website or GAF website and looked up the installers giving you the proposal with certainty every roof install the customer gets customer track record which gets filed on their website if you click on my company name similar to reading reviews from Angie's List ,Yelp , thumbtack ect.. however are properly vetted by the manufacture .
I hope I was helpful did not confuse you in any way shape or form and gave you all information you may be looking for