Half life. Atenolol has a shorter half life than metoprolol, only about 12 hours. Metoprolol can be taken once daily where for full effectivness atenolol should be taken twice daily. People often forget the second dose of anything so in my mind a once a day drug is better.
Beta Blockers. Both are selective antagonists (blockers) of beta-1 receptors. Metoprolol is shorter acting than atenolol, although there is a long-acting form of metoprolol, called metoprolol xl or toprol xl. Otherwise, they are really quite similar.