The border between sci fi and magic is rather thin. While I can imagine a way that doesn't violate the laws of physics to transform the Gang into humans or to transport the rainbowfaces into space as seen in the movie (I have thought about such issues for some of my own works), it would represent a tremendous, to us today insurmountable, engineering challenge, and though it would be nothing out of the blue and rely on extrapolation of existing technologies, those extrapolations would have to go quite far (and if there's anything we know, is that nothing in nature is a linear function, but becomes some sort of curve at some point). So it would still present a leap of faith, it would be essentially magic. Whether the setting has stones, swords or spaceships is irrelevant.
I would say go with where you are strongest. If you have any sort of backround in natural sciences or technology, you could try the sci fi approach. If your main strength is literary prowess, then you can try the other two, and the sci fi approach as well, though with the latter you may leave some grumpy old-fashioned sci fi fans wanting (but those aren't your core public anyway, I think).
