The frame gets the attention; the glass package does the work
Frame material drives a lot of a window's price and looks, but low-emissivity coatings and gas fills between panes are usually doing more of the actual thermal-performance work behind a certified U-factor.
What a low-E coating physically does
A low-emissivity coating is a microscopically thin metallic layer applied to the glass that reflects a large share of radiant infrared energy back toward its source while remaining transparent to visible light, roughly 97% of radiant thermal energy in a well-designed coating.
In winter, that means indoor heat radiating toward the glass gets reflected back into the room instead of escaping through it. In summer, it means exterior heat radiating toward the glass gets reflected back outside instead of passing through. Manufacturers apply these coatings either during glass production at high temperature or afterward as multiple ultra-thin sputtered layers in a vacuum chamber.
What argon and krypton actually add between the panes
Ordinary air conducts heat reasonably well for a gas, which is why manufacturers replace it with a heavier inert gas in a sealed insulating unit. Argon's thermal conductivity runs about 67% that of air, and krypton's runs at roughly half of argon's, meaning krypton is meaningfully better at blocking conductive heat transfer between panes.
Argon is the far more common choice because it makes up close to 1% of the atmosphere and is inexpensive to isolate industrially. Krypton costs more to produce and tends to show up in thin double-glazed units or premium triple-glazed products where the extra performance justifies the price.
A single pane of glass on its own performs at roughly R-1. Adding an argon-filled gap between two panes typically brings a unit to somewhere around R-3, without changing the frame material at all.
Why this matters more than a brand name
Two windows from two different brands, in the same frame material, can perform very differently depending entirely on whether they use a low-E coating, which type of gas fill, and how many panes they use. None of that is visible by looking at the window; it only shows up in the U-factor and SHGC on the NFRC label.
How manufacturers actually apply the coating
One method, pyrolytic deposition, bakes a fluorinated tin oxide layer onto the glass while it's still hot during manufacturing, producing a durable, hard coating. The other, magnetron sputtering, deposits several ultra-thin silver layers in a vacuum chamber after the glass has cooled, producing a coating with stronger performance that has to be sealed inside an insulating unit to protect it from the air.
Neither method is visible to the eye once installed, and a spec sheet or NFRC label, not a visual inspection, is the only reliable way to know which one, if either, a given product uses.
What to ask for on a quote
Ask specifically whether the glass package includes a low-E coating and which gas fills the sealed unit, argon, krypton, or plain air. A quote that's vague on both is a quote you can't actually compare against a competitor's, no matter how similar the frame material and price look on paper.