I'd vote Navigator. While some of my opinion is generalizations, and you might get different mileage out of them, I feel Navigators often make more fun characters. They have a sense of value and superiority Astropaths might lack, with their families, their critical function, and such. To me, they are more likely to act like another Rogue Trader character, larger than life, and aware of their importance, while an Astropath, even a powerful one might not step up to such heights.
Again, these are generalizations of mine, and yours might differ. I've always been more a fan of the Navigators, with what I felt of as RT's ignoring battle-psykers; Astropaths are valuable, and so they often have more powers that don't trick them into bad situations, but I don't like much of their offensive psychic arsenal. Overall, I just like Navigators more. For an especially fun character, you might even let said Nav, if that is their chosen route, grab Navis Scion from Into the Storm, as it gets them some very nice extras, especially if they are one of the Wandering Houses.