I would fly TK only if going east is enroute.
I dont find it viable to use IST to go west TATL from AMS.
TK does have many good points (seat/food/lounge) but i find them to inconsistent: Not every aircraft has fully flat business, lounge can be a zoo and service very surly. At least lufthansa has consistency with angled business and attentive crew. So i think that if you go east then its worth taken up their FFP instead of MM