They seem pretty synonymous or interchangeable. The only distinction that I could think of off the top of my head would be that "thoughts on" would be referring to something inanimate. What are your thoughts on this proposal? Whereas you would use "thoughts about" when speaking about a person. What are your thoughts about Suzy? It just seems sort of odd to say What are your thoughts on Suzy? One other way you could say it would be, What do you think about ...?