The second option is compatible with more browsers. The bind() function is not supported by IE 8 and below, if that matters to you.
Mozilla's page on bind has a pollyfill for the bind function. In my experience it's usually a bad idea to add to the prototype of builtin objects. So the second option is "better" - just make sure to use a descriptive variable name instead of "that" or "scope". Using a common name can get confusing, especially when you add more functions.
