Both are title characters that have blue eyes (sometimes when Stone gets hurt, he gain blue irises) and associated with yellow (two of Stone’s transformations are mostly, if not completely yellow) and have their names start with a S (I think that’s his name, we don’t know the rabbit’s real name).
Both dealt with similar opponents, such as: a fast-food inventor (Plankton and Willie), ghost pirates (The Flying Dutchman and Barnacle Bob), a shapeshifting alien impersonating them/their friends (The Jellians and Melvin the Plutarkin), a red-clad supervillain with their start with a M (Man Ray and Lord Morbad), a villain running a restaurant with a plan to drive down their rival that the protagonist knows closely (Plankton and Bryon), a weird zombie apocalypse associated with eating (The Krabby Patty Creatures and the Living Dust Bunnies), a dark wizard (Planktonamor and Larry/The Lord of Darkness), and evil robots (Plankton’s robot army for example and Cheese & Broccoli).
Both are friends with a scientist that are smarter than them, and they had a part withthe aftermention zombie apocalypse (Sandy Cheeks and Judy Goose).
Both are best friends with someone taller than them (Patrick Star and Andy Wolf).
Both have impersonated heroes with their powers (Mermaid Man & he did use MM’s belt when hewas the Yellow Avenger and Captain Awesome).
Both are my favorite characters.