Many cons I've been at require you to keep your costume secret until the competition.
If that's not a rule, I'll decide based on complexity. Dresses with long trains are just not hall costumes, so I change right before the show. Outfits that are easier to wear might be my daytime costume as well as my masquerade entry. For my next con, I'm wearing a casual version of the character all day and changing into the fancy armored version for the show.