Which is better for acne scar treatment - derma roller / laser?

Depends. All acne scars are different. Seek the advice of an experienced acne scar physician to help treat acne scars with the therapy which is best for you.
Both. Both are great treatments in the right hands. Only go to a dermatologist with a lot of experience, not just any laser or salon that does these treatments. It can be horrific if performed in the wrong hands.