Unless you plan on attending a lot of formal affairs I would suggest going with the belt and the dress buckle.

My reasoning?

Simple, a Mess jacket is never going to make a perfect Prince Charlie. But it can be a very nice formal kilt jacket. I think that a white tux shirt, bow tie, fancy belt, white hose, nice dress shoes, and a semi dress sporran, and a black mess jacket (perhaps with silver celtic buttons) creates a formal kilted look that isn't trying to be something it is not. Also you can use the belt for dressy occasions where the waistcoat would probably be too much.

The same goes for the sporran, a semi-dress one in leather with cantle or part fur is going to be more versatile than a formal all fur one.

If you haven't done so already, you should peruse the "Formal" and "Dressy" Style threads in the "Show us your picts" forum.

Cheers

Jamie