In the Argonautica, both the Planctea and the Symplegades are mentioned. One on the way in search of the Golden Fleece and the other on the way back. The Planctea was also known as, "the Wondering Rocks," and the Symplegades had another name of, "the Clashing Rocks."
Jason (or Iason, before the letter J was invented) encountered both the Wandering Rocks and the Clashing Rocks in his quest for the Golden Fleece. He encountered the Clashing Rocks first. To get through, he released a dove (or multiple doves until one came back), and when it came back with only a few tail feathers missing, heading advice from the seer Phineus, knew if his ship went fast enough through, he could make it. He did with only the very back of the ship being caught between the Rocks. After this, they stopped clashing forever.
On his way back from finding the Golden fleece, Jason came in contact with the Wandering Rocks. He bested this obstical as well and continued on his way.
I looked at both Wikipedia's. On the Planctae's one, it mentions the differences and similarities have been debated, but only talk about the location of them. What are the differences between the two?