Tempting You to Stay. It's a reaction that's both good and bad: Your boss values you so much that he or she offers you a raise or promotion to stay. But it can leave you feeling conflicted: You have another great job lined up__but now that there's more money on the table, do you really want to leave your current position?
You want your intent to be as direct as possible. You are leaving, plain and simple. There's no need to mention where you're going. You can discuss that with your boss (though you're not obligated to do so) and your colleagues if you want, but there's little purpose for including that in your resignation letter. That's especially true if you're moving over to a competitor__you don't want to create ill will in this letter or cause any drama.