It depends on the route you are flying. Long haul I'd say EK F is better- but neither would be a bad choice at all. To give a few comparisons:

Food: QR F menu is same as J class with one or two additional choices. EK menu is different from J.
Seat: QR F will be lie flat. EK might or might not be lie flat. Whatever the type of seat, I find EK seats much more comfortable.
Amenity kit: QR will always give amenity kit in F at any time of day. EK give for night flights only.
AVOD: EK AVOD has bigger choice of films, music.
Lounge: new EK F lounge in DXB T3 is great. Also has free spa treatments. QR premium terminal in Doha is relaxing but not very interesting and only one small duty free shop. There is a spa but its not cheap to get a treatment.