Thanks all, sounds like that if I want to get a RSTV, then the new style is a better bet. DS is built like DD, but even smaller so it would be best for us in the long run to go with that one.
DD has flown many times without her seat - at least 1/2 our trips are to my parents' house, where I keep a car seat for her, so I usually don't have to travel with two. And usually, when we're not going there, DH is with me to help with the carseats. She does great on the plane in just the lap belt. It's the closest to booster training we can do for a while, lol.
We have a gogokidz for one of the seats, so I can always put the second seat in it, but would it would definitely be easier to just have the one. Plus a RSTV would make it easier to do 3 in a row in our third row when needed since the middle seat is lap belt only.
Anyway, thanks for all the input!