Child Custody!!

I have 3 children to my ex husband and they all live with me and their step dad, step-son and new baby brother for the past 4 years.  We live 3 hours away from there father and they visit him every holiday and long weekend.  My ex has now said our 10 year old daughter wants to live with him.  I have discussed this with her and she has said that she would live with dad cause she likes the country (where he lives).  We do have different parenting ideas and i have never wanted  the kids to be split up.  The other siblings are 12 and 7 which havn't indicated they are wanting to live with him.

We decided together she would finish primary here and then we would re-asses at the end of the year.  However my ex does not like this decision and it has caused heated discussions between us.  My daughter says she wants to live with him but i feel she is just being torn as he is telling her she gets to decide.  Help what do i do.  She is a wonderful girl and we have a fantastic mother/daughter relationship.  But please if you have any suggestions i would be greatfull!