I always wondered about this, and this CNN article posted today reminded me to post this poll Wife seems to prefer a male voice, but is ok with the female voice (which is what I use).
In my car, the helpful voice for commands is male. "Voice activation system...I couldn't understand you. Please repeat."
The car nav system directions are provided in a female voice. "Take your next left, [idiot]...make a legal U-turn, [dummy]...[are you deaf too?]"