At our local competition, we have to play 2 pieces with contrasting styles. The range is really big, with some playing Haydn G major, up to Bruch and Lalo. I’ll be playing Mozart 4 with the Joachim cadenza first mvt, and deciding whether to play Bach d minor Sarabande, or Tchaikovsky canzonetta, Thoughts?
Unless there is some huge local preference (i.e., not wanting to hear Piece X ever again), go with what you can nail better than anyone else.
I'd say the Tchaikovsky is a bigger contrast, but I agree that the best choice is the piece you play best.
If you play both equally well, then you might also want to factor in the pianist. Is the pianist experienced, and are you comfortable playing with that pianist? The piano part in the Tchaikovsky is not technically hard, but some less experienced pianists do not understand how to follow a violinist's phrasing or rubato, and that limits what you can do in a slow movement. If those could be issues, pick Bach to diversify the risk.
I think the Canzonetta is a much better choice because it grabs the heart. Solo Bach is great but I do not recommend it in a situation where the judges are mixed (i.e., they are not all violinists) because pianists, curiously, have very little appreciation for how hard it is to play solo Bach on the violin even though they play tons of solo Bach themselves.