Until age 2, our dd has been "infant in arms" (dh is cheap).
Now, we will bring her seat with us, because we've tried the airplane lap belts with her, and she can undo them easily. Dh is a pilot and is less worried about a crash (which would be statistically unlikely and could be so awful seatbelts wouldn't matter anyway) than he is about turbulence or a rough landing because of a weather condition, pilot error, or an ATC oversight. He'd like dd belted in securely for takeoff, landing, and any time she's not nursing.
If you choose not to bring your seat, and you're renting a car at your destination, most rental companies have seats you can rent for a fee (no infant seats--I think they only have convertible or forward-facing seats), and you have to reserve them ahead, generally. If you want to make travel easier, you can check your stroller at the gate and get it as you exit the plane: there's no additional cost.