No need to guess since they are all the same...
Best way to buy double glazing if you do want to buy from a big national company is get rough measurements yourself, an idea of what type and design of windows you want, then spend an afternoon or two going round showrooms. Tell them what you're doing, ie going round all their competitors, examine the windows while they tot up their best deal to make sure you're getting what you want in terms of quality. Say thanks, leave with the quote and go to the next place.
You'll find you'll get the 'best' price without the sales hassle. Once you've decided on price/quality which company to go for, invite the surveyor round to measure properly. You should never pay more than 50% of their list price or you're not getting a good deal.
NEVER invite someone into your house in the evening!
Buy on credit where possible (some may have an extra incentive for you to do so, but make sure you can pay off instantly without penalty) as this helps if the company goes into receivership (2 of my 3 final choices did despite all having traded for 25+ years)