Old question but here is the answer... there is none.
The movie officially leaves it to be ambiguous if it was Bianca or Latias. Online does favor Latias but there is a decent amount of evidence that it was Bianca.
1) The audience is purposely shown that Bianca left her hat behind before the kiss. Why show that the only detail of difference between the 2 is now no longer on the real Bianca if it didn't mean anything
2) The girl was visibly somewhat out of breath after a short run to catch up to Ash and the gang which is something that Latias never showed previously in the movie after being chased multiple times.
3) The song before the kiss is called Canon(music) which is similar and pronounced the same as Bianca's Japanese name of Kanon(sound of flower).