Batman and Superman are the A-Listers of the DC Universe, so their inclusion alone - and the fact that they will be on the big screen together for the first time - guarantees the movie will be a hit. The fact that Marvel has managed to get their hands on Spider-Man is a game changer though, and while the mediocre movies starring the character in 2012 and 2014 have dulled interest in him somewhat, seeing Spidey in the MCU is still a huge deal. Focusing marketing materials on Captain America taking on Iron Man is significant enough, but when you throw Spider-Man in there, it's not just comic book fans who will be counting down the days. Batman v Superman doesn't have a similar A-List addition and will have to rely on the appeal of its two leads alone. Moviegoers are already very familiar with them both of course, so will the same level of interest be there? It's hard to say at this point...
