Is atrial fibrillation or flutter a more serious condition?

Same: They both are risk factors for stroke and can cause similar symptoms. The main difference is that flutter is easy to cure by ablation.

Both can be: Atrial fibrillation and flutter are both common arrythmias. Flutter tends to degenerate into fibrillation; the difference is that flutter tends to be regular and fibrillation irregular. Generally speaking, irregular is worse than regular because there is a higher risk of clots forming in the heart which can then cause strokes and other potential problems. Most flutter will become fibrillation.