2
They read very similarly to each other but I am sure they are viewing the cardiac cycle from different points.
2 is to explain atrium and ventricle are contracting alternatively(they are not in systole at the same time).
1 is to explain there are three distinct periods in the cardiac cycle: atrial systole, ventricular systole, both diastole.
Only top voted, non community-wiki answers of a minimum length are eligible