The first one is neither - it's string.join(iterable). If you keep that in mind then the design becomes clear - it would have been weirder to force every iterable to have some string-related method. Better to put it on string, where it more reasonably belongs.

The second one is the typical OO pattern where you ask an object to perform some operation on itself. This is how split works in just about every OO language - if you have trouble remembering it, it might be helpful to remember split can be called without any parameters - in that case your alternative variant won't make any sense.