I am uncertain as to what difference the various wool weights make to the end garment.
Others can describe the difference in construction, resistance to wear, resistance to wrinkling, or other factors. All I will say is that I have a very lightweight casual kilt as well as a heavy (16 oz) 8-yarder. The difference is huge. After wearing the 'tank' for a while, I can put on the lightweight casual kilt and it feels like I'm not wearing anything. I've come to prefer the substantial heft and feel of the heavy wool. That's personal preference, I admit, but it was totally unexpected. I live in a hot climate (South-Central Texas), and I was expecting to be turned off by the heavier wool. So I have shocked myself by taking a fondness to it. YMMV.