What is stronger 19mg of atenelol or 10mg of nadolol? Or are they equivalent?

Not equivalent. Both atenolol and Nadolol are beta blockers. However, inside the same class of medicines the drugs do not have mg by mg equivalence. Also: dosage of 19 mg of atenolol does not exist.
Beta blockers. There is no such a dose of 19 mg atenolol. 20 mg nodalol is equivalent to 25 mg atenolol.