I want to get double pane, energy efficient windows for my home. How important is Argon gas a factor?

I am considering replacing my aluminum single frame windows with double pane energy efficient windows.
One of the manufacturers I have contacted says “I don’t put Argon gas between the panes because the gas will leak out in a year or two”. How important is Argon or another inert gas between the panes a far as energy efficiency is concerned? Also, some installers take out the entire existing aluminum frame while others insist to remove the entire frame is not a good idea and they leave the frame and install the new windows over the existing frame. Which installation method is recommended?