Harris, no question. Sanders is way too old and also I feel would not be a very good executive. Warren has a lot of baggage that would make her a weak candidate.
Lack of experience is a positive for Harris, not a negative. Anyway, she'll be more experienced than Obama was.
No, it will be equal. Both served four years in the senate. Obama was also a state senator for seven years while Harris Attorney General of California for five years. But she would have more life experience in 2021 at the age of 56. She would be one year older than Obama is now.