I would wear any of the first 3 here http://www.usakilts.com/sporran_day.php as semi-formal at any time of day. I'm not big on a lot of flashy nickel.

When I step up to formal though, I'd pull out the sporran with a little more "bling" http://www.usakilts.com/sporran_dress.php

but as is often said, there are no hard and fast rules, so whatever you think looks good. After all, odds are, no one else in the room is going to have a clue about highland wear.