I feel like it's a toss-up of which eevolutions to use. From what I have seen and heard from locals you just have to notice what decks people seem to play the most when debating on which eevolutions to use.
However, for larger tournaments like league challenges and regionals, you have to use your best guess on what people are going to be playing, since there are just so many people at larger tournaments.
Then there's your own deck to consider. What if you already use a Stage 1 Water deck? Why would you even consider vaporeon, for style points? You're just going to have to look around at recent forum posts and see if any local tournaments have posted the winning decks and make your best guess, because tomorrow is the first League Challenge that I know of that BREAKthrough is legal in.