Whether or which
Most marketers are busy trying to persuade people to buy their product. Confusion sets in, though, when you compare a pitch designed to get someone to buy any product in the category (you need an mp3 player because you can listen to music) vs. buying your product instead of the competition (ours is cheaper and bigger and better).
Are you trying to make the market bigger, or just grow your share?
When competing against a market dominator, your marketing generates more bang for the buck when you try to steal people who have already been persuaded to enter the category by the other guy. This is the Newton running shoe story. Nike sells fitness, running, camaraderie, effort, glory. Newton sells "buy us instead of Nike."
It doesn't pay for an insurgent energy drink to sell "thirst" because much of that marketing will just get people to go buy the brands they've always bought. The opportunity instead is to provide leverage at the last possible moment in the buying cycle.
Getting new people to enter your market is hugely expensive. There's no way I can persuade a non-book buyer to start buying books–I don't have enough time or enough money.
This thinking rarely grows the market, though, so it falls on the market leader to figure out how to market well enough to get people into the category itself. The critical issue is to decide which one you're doing. Are you working on whether or not someone should buy, or on which one they should buy once they realize a need? Do your employees have the same answer?