Atenolol is a beta blocker, but its half-life is far longer than what is the case with metoprolol
(6-9 hours vs
3-4 hours). So extended release is not quite as necessary.
Some beta blockers are able to cross the blood-brain barrier and some are not. Propranolol and metoprolol are fairly able to do so, but Atenolol is, as far as I know, not. Which means, propranolol and metoprolol can provide more cognitive side effects (forgetfullness, confusion, mood disturbances, etc.) than Atenolol.