Is a fast or slow heart rate healthier?

Slower: But everything within reason. Neither too slow nor too fast is good for you but within the range of normal, slower typically is associated with being more fit (although it's possible that some medication or electrical conduction defect could lead to artificially slow heart rate).

Depends: It depends what you are doing. At rest a slow heart can simply reflect good physical fitness. A fast heart at rest may signifysome underlying rhythm abnormality like atrial fibrillation. However, during exercise a slow heart rate may reflect pathologic chronotrpic incompetance while a fast heart rate is a normal, healthy response.