We've flown twice with our DD#2, and once with DD#1 in our laps. We were fine. There was a little squirming at the start, but when they realized it was sit in daddy's lap and play time, they were ok - except for one or two attempts to break outta jail. Both daughters were 'daddys girls' and loved to sit in my lap and play so it wasn't too bad.
We also have traveled with a car seat, and it's a tradeoff. Child is easier to manage, but one of the trips DD#1 did nothing but scream and try to take the seatbelt off - even though she was a perfect angel in the car in the seat.
We've done both. You will be fine. Bring toys along for the infant to play with. New toys are a good thing. We also had colors and I read some books to her.
And of course, I'm beating all those odds. In the 30+ times I've flown to WDW and other places, I've never experienced turbulence that would bump me from my seat. And to be honest, I would assume the pilot would know ahead of time of bad turbulence, would put on the seatbelt light, and I would have asked for a seatbelt extension to hold me AND DD in. I know they have them, but never had to use them. (and I'm sure there will be people that post here that they had turbulence out of the blue/etc... so take my experience for what it's worth)
You'll be fine.