The Semi Trad is warmer than the casual b/c it's got (on average) 2 more yards of material in it than the Casual.

Is PV good in the winter? depends on what you're used to and your conditions. It's NOT better than wool, but it is better than cotton or cotton poly. If it's the only kilt you have, then wear it in the winter! If you have a PV and an 8 yard wool, then the wool one will probably be your "standard kilt" for winter months.