There are many products out there. When someone goes on and on about how their product is the best out there, ask what other products they offer. If they cannot offer you a selection and say, they get better prices by using one Manufacturer, this should be a red flag.
For example, I once heard a Vinyl Window Consultant say his product was better than fiberglass windows and doors because his were fusion welded and fiberglass used manual fasteners. Fusion welding is not better than Manual fasteners; this point was irrelevant, but it got them the job at a huge premium; the customer wanted Fiberglass but settled because of this recommendation. The customer also did not want white windows, but still went with Vinyl because of what the rep said (Painting Vinyl is possible but it voids most window warranties especially if you use a dark color, plus it will not last as long as painted Fiberglass that was meant for painting and came in many colors factory painted.)
What the Homeowner should have been told is, Vinyl is plastic and is popular because the price to make them is much less than fiberglass or wood windows. You cannot melt fiberglass or wood together (Fusion Welding) just like vinyl windows do not hold screws (Manual Fasteners) well because they are hollow. Since you want to paint your windows dark grey, wood, fiberglass or even aluminum windows would be better for that application.
As a Product’s Quality can be oversold, so too can the skill of the labor. Do your research and do not make a choice until you have all the options.
By the way, better prices because of volume discounts or lower overhead do not actually change the job’s cost by much. Also, vinyl windows will almost always cost a contractor less than fiberglass, wood, or aluminum clad windows, and have a faster lead time once ordered. This gets them paid faster.