They have a little bit different meaning. When Thai people say ภัตตาคาร, most of the time it refers to Chinese dinning or Chinese restaurant (and it usually expensive one) . On the other hand, ร้านอาหาร means restaurant; any kind of restaurant including cheap side street restaurant to expensive restaurant in the hotel or the mall. To be safe you can refer to any kind of restaurant by saying ร้านอาหาร. I hope this can help you.