A is sincere in a sense they are faster in getting back to me with offer as the HR was in the interview and they make the decision straight, right now SMSing the HR manager and talking to her via SMS.
For B, they have to go through their boss approval first, then they can get HR to draft letter of offer, then HR's boss to approve, then finally get back to me in about a week.
That's why I very torn now... I prefer B but I know my pay will be lesser.