Resignation letter

I am currently working with some company and I have spent one month (training period) in the organization (my probation period is of 90 days). But now I have got better opportunity un my dream company. They are ready to give me to serve notice period in my current organisation. I have read the clause of offer letter of my current employer that if I want to resign during probation period then I have to serve 15 days notice period. So now 2nd month started with my current employer and now I am on floor or hit the floor. So now I want to give resignation from my current company so please suggest me should I give them the same reason to leave this current organization for better opportunity or shall I give them some another reason to leave this company like personal issue etc. Please suggest me. Abd is there any possibility that my manager will disagree on this and he will refuse me to accept resign. If I will get resign so shall I eligible to claim the relieving letter from the current employer? Please help me I am very confused.