Slash Film has looked into which website URLs Warner Bros. owns that track with those titles. The site reports that they own both SupermanVsBatman.com and BatmanVsSuperman.com, plus BatmanVsSupermanMovie.com -- but notSupermanVsBatmanMovie.com. So the current best guess is that the film will be called… Batman Vs. Superman.
But hey, either title is great. Simple and evocative. And I, for one, can't wait to see Batman kick Superman's ass!