I'd go 1/4" or 5/16"
The C-channel that was on my truck was 1/4", and the PO said it worked really nicely, and had reciever holes in front and back which he used for mounting his winch too.
If your using 4X4 steel then it would be alright (in my mind atleast) to go with 1/4"
Heck! some people use 3/16". I think that would be safe in my mind aswell.
the best thing about going straight and simple is that if you do decide to with 3/16" and you beat the hell out of it. It would be easy to replace if you made up the brackets easy to transplant.