What we did was use the car seat. And we put a member of our party in front of the seat. In our case we are a family of five. So I and DD (age 2.10 at the time) sat in one row with the carseat being next to the window (where it is required to be). DH and the other two children took the row in front with my middle child sitting in front of her sister. Than any "seat kicking" was done to her sister and not a stranger. DD fell asleep on the way home right after take off and slept the whole flight..it was heaven! She had plenty of room for her legs and we could even use the tray (it wasn't completely flat but flat enough to use). It really depends on the airline and plane you get in terms of seat room. I did see another child on the plane about the same age as mine. She did not have a carseat and she was a mess. She was trying to sleep but couldn't get comfy. She was then trying to get up and out of her seatbelt. Her poor father was sweating trying to keep her under control.
We also used ME and no rental so we didn't need it..except..DH's boss happened to be there the same time we were. He was staying off site and came to get us one day to spend some time with them at their resort. Good thing I had her carseat!