Trying to keep this as brief as possible, in the UK there are basically 2 styles of kilt jacket, the Argyll/Crail and the Prince Charlie. The Argyll/Crail are almost identical, the difference being in the cuff styling (the Argyll has a gauntlet cuff and the buttons run round the cuff, whereas the Crail has a straight cuff and the buttons run up the sleeve).

The Argyll/Crail is worn as day wear (tweed, usually blue, green or charcoal grey - equivalent of sports coat or suit) or semi-formal/formal (black barathea). The degree of formality is signified by the accessories worn (plain leather day/semi-dress or dress sporran, plain or bow tie, etc). The Prince Charlie is normally black barathea and is always formal - usually worn with a waistcoat (vest) and with the higher waist kilt where the top is covered by the vest - in this instance a belt should not be worn.

If you're only going to buy 1 jacket, I always recommend the charcoal grey tweed Argyll/Crail, which when worn with the right accessories can be dressed up/down to cover most occasions except the most formal.