Tartan Tweed kilt

[Nanook]

Well, just like worsted wool, you can find Harris tweed in various weights.

Harris tweed comes in not just an assortment of weights (although the real heavy stuff has become quite rare as demand has nearly vanished) but also textures due to the dyes, wool and weaving differences.

seems you can't find heavy weight saxony tartan any more).

I assume you mean wool spun tartans? The army has now-- for reasons of cost-- switched over to (a unified) worsted tartan.

The pleats will be softer, and the kilt all around has a softer feel.

Wool spun pleats are not really softer but I'd say less crisp. For mechanical wear, robustness and protection from the elements there is hardly a finer material than spun wool. It is these days more costly to make due to advances in automation, less smooth to the touch (what some might call scratchy) and patterns/colours are less well defined. Other than Hainsworth or Robert Noble I'm not quite sure who has even made wool spun kilt cloth since the 1950s.