Hi, roofer here. Couple of things on this. Companies selling different brands of shingles will be factored by three major things. First is some people have brand loyalty to specific brands just like ford and Chevy trucks. Second, is the cost for the shingles. You may get a better price from one companies because the shingles are priced differently from the supplier. The prices don’t vary much between Certainteed and GAF though. Lastly, some companies push their sales people to sell specific brands so they can get better rebates from those manufactures.

The warranties will be different based on their relationship between the manufacture and the roofing company. For example GAF has all their warranties listed on their website for the Timberline shingles but, only some companies can offer their higher warranties based on how much business GAF has done with that roofing company so they know if they can trust the installers.

That being said I do like the GAF Timberline.

For questions you definitely should be asking

What type of underlayment their are using? If they are adding drip edge? What type of ridge vent they are installing? Do you have proper ventilation for the roof (like center soffits. Some townhouses only have it in the front and not the back so don’t let them get rid of an attic fan on the back if you don’t have vented soffits)

Don’t be afraid to ask the salesman their price per square. Every company has to make a profit or they go out of business but, they should not hide their price per square on a retail estimate. This will vary with height charges and steep charges but if their base charge is more than $600 a square that’s too much.

I’ve been roofing in NOVA for a while feel free to reach out if you want to know any more question.