If you want my opinion, if you can admit Mozart's Cadenza is better than yours, play that one. I understand the will of a composer to get his own works known, but a Classical concert with a symphony is, IMO, definitely not the place to do so. People will judge you on your performance, and playinga really good performance of your own compositions will, in general, get a less good review than to play a top notch performance of an actual piece of classical music. You just won a competition, probably many important people will be there, and you better get the best impression as a pianist, not as a composer. It is true that the best impression you get at this concert, the more you have chances to be able to play your own compositions later. As of what to wear ... a formal dress will do the thing i guess ... just choose a nice, straigth one and it will be fine.