obsidian seems to be some huge ambitious app that means to do a bunch of graphing and whatnot. seems to be more of a workboard app than a note taking one. (just going off their homepage. i was not familiar with the app before you mentioned it)
joplin is just for taking notes or journalling. its very good at giving you built in tools to organize your notes as well. but its quite a minimal and efficient style. it is a powerful app. but it's made for a different purpose.