Answer by Japanese teacher
Both expressions are suitable to say “I’m getting better”. だいぶ daibu and ずいぶん zuibun mean “a lot, much”, so you can omit this if it’s not applicable according to your situation. おかげさまで okagesamade is a set phrase usually used to thank someone for his concern or help. You can also follow up with a ありがとうございます arigatoo gozaimasu at the end to further express your thanks for the concern shown.