The aluminum vs steel is just the bolts and nuts, depends what you have on your truck, My SLE had steel so I went with the steel kit RC sells, easiest way is to take a magnet to it to check like "emeryz28" mentioned.
Haven't had any issues with the 2in level, makes the truck look better.
This should answer all your questions you may have - http://www.gm-trucks.com/forums/topic/153363-2014-leveling-kits/page-1