A lot is just whim/mood. I love the convenience of the casuals - easy on and off when that is something to consider. Also love the pleats in the semi-trad...not like my 8 yard hand sewn kilts but still very nice and much less expensive than hand sewn

I had my 5 yard wool kilts made for rafting the Grand Canyon...to take up a bit less space and weigh less than a hand sewn wool.

My wool kilts are usually in tartans that are meaningful to me. The poly viscose are usually in less meaningful tartans but purchased because of what they mean to me - or love of the look of the tartan.

Executive summary: I have no clue...I'm just a kilt addict with a craving.