I live in a similar climate to Scotland (not the north pole, but not tropics)... my advice would be pretty much the same as the others with 1 qualifying question:

Are you normally a "hot" person or a "cold" person? Meaning... do you normally find yourself complaining about being hot or cold compared to others around you?

I am a "hot" person... I'm always warm when Kelly is cold. I wear 13 oz kilts year round. I also HAVE 16 oz kilts, but they see more "light of day" in the winter time. Bottom line... I SWEAT in them in the summer.

Also, as others have stated... the lighter weight is better for dancing and the difference in swing between the 13 and 16 oz isn't THAT great that it should deter you from either one.