It may not make too much difference in your selection, but I think the standard frame decking material is actually treadbrite instead of diamond plating. It does save some weight, but actual diamond plating material I believe is a little more rugged and maintains its flatness a little better (ie - doesn't stretch/warp)
I used 3/16" diamond on my frame which may have been a bit overkill, but I can stand, boogie and crush cans without fear of warpage.
If you're in the Denver area, I'd suggest looking at Alreco over in commerce city. Bob takes cash only but I picked up some "used" material there and it was fairly reasonable.