Apollon actually wasn't considered a sun god in Ancient Greece until much later. Him being worshiped as a sun god came from later Roman influence, most likely them syncretizing him with an older Etruscan god that had a similar function. Apollon in Ancient Greece was actually worshiped as a god of music, health and divination, whereas Helios was god of the sun.

You can see more at here for Apollo and here for Helios.