They both work and mean almost the exact same thing :)

verb To drop, toss, or set someone or something heavily or clumsily (on or into something). I plopped Sally onto the table so the doctor could have a look at her. When something like an ice cube drops in the water it makes the sound of "plop". To use plop as a verb means you drop sth down and makes that sort of sound. Plunk is nearly the same.

Yes both mean the same thing