it seems to been with in a couple bucks between the colors for for 3/16th at 24''x36'' and the thicker seems to be cheaper for translucent than for the clear
If I may be so bold:
If you are using it for a window only there is no reason to throw away hard-earned money. 1/8" is great for windows, as is 3/16.
3/16 is the thinest gauge that I would use for any type of structural support, it is the gauge that I use a lot of. It does an outstanding job when all angles are 90 degrees, and there is no shearing force involved.
1/4" is very rigid, but will add up quickly when it comes to weight.