What is more serious, atrial or ventricular fibrilation?

Ventricular. Both are serious. However, vf is often associated with pending collapse of the circulatory system. Vf is a serious complication of acute heart attacks and other heart disease and must be treated aggressively, usually in the ICU setting. Af can be life threatening, but is often discovered incidentally and some live with it untreated. Untreated vf is usually fatal.
Arrhythmia. Atrial fibrillation is a rapid irregular rhythm which retains the ability of the heart to contract. The ventricular fibrillation is a rhythm which results in death if not resolved to a better rhythm.