You know, taking the dichotomy you propose as accurate, I'd go with the sleazeball hands down. You might not like them but you can work with sleazy people if you know what they are. They are simply pursuing their self-interest and respond predictably according to realistic calculations of where that lies.
A narcissist on the other hand you can't work with on the basis of realism because he's not rooted in the real world. He operates in a fantasy world. A sleazeball won't act in a way that harms himself but a narcissist, while every bit as self-oriented and deceptive will, and then go looking for scapegoats, even when that does more damage. A sleazeball only scapegoats when it's to his advantage.
So would you rather deal with someone who is rational but selfish, or someone who is unpredictable, self-destructive and selfish?