The gravity difference affects his voice quite a lot. Taking into account the different instrumentation, I like the space version of his voice more, presumably there was more fluid pressure in the larynx and sinuses and, perhaps, different forces acting on the lungs and other organs involved in sound production and modification. The atmospheric content and pressure are much the same on the ISS and at sea-level on Earth, apparently.
Here is another interview at the BBC on the same day, where he shows his adorable geek side, covers many subjects (very little about the song) and answers a few viewer's questions:
"The loneliest people live in cities."