I used the seatbelt, too, for my son ever since he was 18 mo old. Of course, one leg of the return flight home, we were "those people," and we neded up having to hold him. But we've never had a problem w/ just putting him in a seat. So I have to agree w/ the other postings that I would just use the regular plane seatbelt for a preschooler.
My concern is really just holding the baby. I would normally get her a separate seat, but I guess I'm just being cheap right now. I saw a special once on the Sioux City, IA, crash from 1989, and they said that every child who was secured in a seat lived. The children who were being held did not. With the harness I am looking at, you hold they baby, and they wear a harness that straps to your own seatbelt. That way, too, in turbulance you aren't playing bouncy baby throughout the aircraft.