Aluminum Sliding Windows/grilles

[Noodler 0]

Hi all, which is a better material: Single or double hollow?

I have requested for a few quotations for the aluminum sliding windows/grilles (just for service yard), as follows:

Quote A < $700 - Double hollow

Quote B Mid $700 - Single hollow (claims to be factory price)

Qoote C > $800 - Double hollow

If double hollow requires more materials, why is single hollow more expensive than double hollow?

Would appreciate if anyone can recommend their contractor if they are satisfied with their works.

Thanks!