My name is Mallus and I disapprove of this message!
Seriously though, I've always found that my players relate more to the characters and their motivations than their trappings. A peasant can be less believable than a laser-equipped shark, depending on the quality of the characterization.
Maybe what I'm trying to say is that it's better to strive for psychological believability. Consider Buffy the Vampire Slayer. If you examine the show'ss trappings; SoCal vampires, suburban demons, 98 lbs kung-fu schoolgirls, eventually, robots, it looks like an utter mess. Completely unbelievable. Then look at the way this freakshow is written, characterized. Suddenly, the show becomes one of the more realistic and effecting coming-of-age stories on TV.