I don't see either being the biggest song of the Summer. I believe SOTS hasn't been released yet, or isn't known by the public slightly yet.
'All About That Bass' never built up steam. It literally just...exploded 1 day.
'Shake It Off' was a #1 immediately after release.
'Blank Space' was a #1 hit almost immediately.
'Uptown Funk' just blew up within literally 2 weeks.
'See You Again' rose in 1 week following the movie premiere.
The point I'm trying to say is that the biggest hits recently didn't build up steam over months, they just immediately exploded. Thinking Out Loud is one I can think of recently that took months, because it was hovering on iTunes ever since the album's release in June, but overall, hits have been rather exploding out of nowhere or coming with a major artist releasing them the same week. I think Song of the Summer will follow that path, be a hit that random explodes, or a new single from a major artist.