This is second-hand advice from my sister who took her 1-year old to Hawaii last month. On the way there they had 3 coach seats. On the way back, 2 first class seats.
Her impression was the F seats where better, because they had more room to maneuver, and the baby wanted to be held for over half the flight time anyway. They also had to lug the car seat through the airport on the way there. Keep in mind that they had 3 flights, so this may not apply on a single nonstop flight.
It would seem to me that safety is a separate issue to that of convenience/comfort, and I won't comment on that.