The 1/8-inch-thick bracket seems more than adequate. It's just as rigid with me standing on it as the 1/4-inch version, but it weighs ~2/3 as much (making it cheaper to ship) and is easier to build. This test indicates that it's plenty strong since I changed a tire lifting the truck by the step bracket with the step installed. After I lowered the truck, removed the jack, and stood on the step, it dropped less than 3/16-inch.
See also:
http://www.thedieselstop.com/faq/9497faq/tsb/tsb/95-08-06.php3 
