In Breakthrough Advertising, Gene Schwartz writes,
Every product appeals to two, three or four . . . mass desires. But only one can predominate; Only one is the key that unlocks the maximum economic power at the particular time your advertisement is published. Your choice among these alternate desires is the most important step you will take in writing your ad. If it is wrong, nothing else that you do in the ad will matter. This choice is embodied in your headline.
This is why testing is so important. With email lists, this can be done at fairly low cost. Given three headline choices, send each one to a third of the list and test the open rates. Go with the headline that receives the most response.