I find that the hottest part of a kilt outfit isn't the kilt but the kilt hose. As I've said in other places on this forum, we wear 16 oz band kilts for parades and competitions all summer, and it's the socks that are the killer. Air circulates under a kilt, and I find that I really sweat only under the belt, which would be true regardless of kilt weight. On a really hot day, I do find my 16 oz kilt to be a little itchy on my thighs, though, and I just wear a pair of bike shorts.

10-11 oz tartan makes a kilt that wrinkles easily, and the pleats don't hold a press nearly as well as a heavier weight kilt. But it does tend to be "smoother" and less itchy in hot weather. It's also very common to have 10-11 oz tartan that doesn't have a kilting selvedge, so many kilts made in this weight need hems (not universally true, but very common). This is actually neither here nor there in terms of how a kilt looks, but some people don't like hems in their kilts. And a 10-11 oz kilt just doesn't swing like a 16 oz kilt does.

So, when someone asks me, I typically advise 11-12 oz for dancers (a heavier kilt is a lot to jump around in) and 13 or 16 oz for a guy. But, as with anything else, it's just advice.