Reasons for Latias:
1. She liked Ash.
2. Bianca could have made the sketch for her to give to him.
3. No voice/hat.
Reasons for Bianca:
1. The kiss and the painting were thank yous for saving the city and Latias.
2. Leaving in a rush to flag down Ash and give him the painting, she forgot her hat.
3. In the credits, Latias is seen flying over the gang as the boat departs. If it was Latias she would have to find a location where she could transform back in her pokemon form, fly off, and meet the two baby Latios she was flying with over the gang within a matter of seconds. Bianca however would have had no issue with this.