well it depends what the nature of this 'festival' is, and the role you would play in it.
Per se: If it's a really serious festival, and you're playing for serious musicians, then the shostakovich prelude and fugue seems the most musically substantial. Then again, that's not saying much, as of all the russian composers i love, shostakovich does not rate too highly. If it's just a festival for music lovers, and you're playing to a general audience, Chopin would be better - however superficial much of his music is, the fact that Chopin's works are extremely 'accessible' means most musicians (including myself) are forced to play him pretty much out of necessity in our programs to keep passing our exams, earning our bread, and so on.

. Not that I dislike Chopin, I like some of his works - but I prefer a deeper, and often darker sound world myself.
However, if they're after an advanced level
romantic/contemporary work, shostakovich doesn't exactly fit the bill, so in that case, you'd have to go with the chopin. In this instance, I'd play what you're most comfortable with.
If possible, I'd eschew both shostakovich and chopin... Looking at your pieces... let me suggest... Etude-Tableaux Op. 39/1? If you want something slower, how about Rautavaara's Piano sonata No. 1?
Regards,
thetamman