Possible comment/question I might use if the right situation presented itself:
"I used to work with a JW and he always used to say that the preaching work you guys do is a 'life-saving' work. Could you explain exactly what that means for a regular guy like me who tries to be a good husband and father, lives by Bible principles and is an all-around good citizen, contributing member of society?"
Don't let them off the hook by saying "God is the judge of who will survive Armageddon." If the vast majority of Armageddon survivors will be JWs, then it's a life-saving work and your God's a jerk. If Jehovah is going to read people's hearts and let in a sizeable percentage of non-JWs, then the "life-saving" aspect (and also the urgency) of the preaching work goes right out the window.
Can't have it both ways.
om