I really don't think you can go wrong with either a 13oz. or 16oz. tartan if you're getting an 8yd kilt. I've worn both and don't think there's that much difference in terms of heat. I tend to focus more on the additional weight that comes from the large amount of fabric contained in the pleats (one of the reasons that I'm such a fan of Matt Newsome's low yardage knife-pleat kilts!). 16oz. fabric will probably hold up better in the long run, although you may be limited based on which tartan/color scheme you decide on.

If you're planning to join and participate in a clan society/ association I would encourage you to buy and wear a kilt in that tartan. Normally folks will join a clan society/association that they have a genealogical tie to.

Cordially,

David