Both 5052 and 6061 are commonly available in sheet form. The Yield strength of 5052 is only 28KSI, where 6061 is 40KSI. The 6061 is easier to machine, you can weld with either.
Unless your worried about corrosion resistance, stick with 6061 sheet.
We typically use 1/16" plate (0.063) unless there is a need for something heavier. If your bending 0.125" sheet you will definitely need to have a larger bend radius on your sheet metal brake to not induce stress cracking.
http://www.americanmachinetools.com/bend_radius.htm
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