I would use aluminum for some of the skids like the tank but nothing beats a solid piece of steel under the belly when you drop off hard onto a rock with the full weight of your rig!
I kind of agree here (sorry, didn't add what I thought of aluminum earlier). The main reason I don't want aluminum on mine is that it is not fun to work with. Lets be honest, we are talking about armor. Its meant to beat on so you need to be ready to weld and fix it as required so it can continue to protect your rig. Steel is easy to cut, weld, heat, bend, ect. Aluminum on the other hand cost more and requires special tooling to work with. Sure, you can weld it but it will require either a spool gun for a GMAW (MIG) process or TIG attachment/welder. Your talking big money for both and considering your already spending more initially to buy the armor, I just have to ask why? Also, have you ever tried to heat and bend aluminum? It is all together different then steel. Its very easy to make your plate weak at which point your kinda screwed. Sure, I understand lighter is nice but seriously how many times have you been out wheeling and said that you wish your rig was lighter? Enough to warrent the added cost and pains?
I'm not sure I'd even use it on corner armor as I want to weld on tube fenders to double as an upper rock rail to protect my tub where to top seals.
In the end its your build. Do lots of research and make it how YOU decide is best for the wheeling you want to do. For me, that is highline, stretch, belly up skid for clearance, linked and locked with lots of armor. That doesn't mean it will be what is best for you.