<3 Sheet Metal Chassis <3
Well I prefer to use 1/8" aluminum for the bigger, longer outside frames. Reason is that these are your main frames for your chassis, you don't want to lack of any strength in it.
I used 1/16 for smaller parts in my chassis that weren't going to be big enough to worry about too much strength, so you can feel free to use a mixture of any sizes.
I prefer to use Triangles as lightening patterns. They keep your chassis strong while easily
Here are two examples of my chassis' using 1/8th material and triangles. The first chassis was actually built as an of season project, and was a very good and strong chassis.
Something additional that we did to that chassis was get the corners of each flange welded shut, that way it was harder for them to come apart and they were very strong.
Also another thing I have learned is that reversing the outside plates flanges, to go outwards rather then inwards. It makes it easier to assemble (because you don't add flanges on the side of the front/back plates), and becomes slightly lighter (again because of the flanges you remove).
Here is an example of that
Have fun with your chassis design and make sure to post your results!