Termopane windows or double windows?

I have a large single-pane window in my den (side-by-side slider) that transmits road noise.
I thought of replacing it with a new dual pane (thermopane) window to reduce noise transmission.This means ripping out the old window & frame.
Another route to go is to place a duplicate inside window on the inside sill of the existing window, like an interior storm window.
This site claims that a double-window is superior in suppressing noise than a single thermopane window. http://www.soundproofwindows.com/comparison.html
Is this only salesmanship? Any opinions?
--
Walter
www.rationality.net
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Walter R. wrote:

Yes, but it is less effective at keeping you cool in the summer and warm in the winter. Of course just about anything will be better than most sliders.

--
Joseph Meehan

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