We fly every month, sometimes taking two trips and it is indeed an FAA rule that they cannot be in a sling during takeoff, landing and any taxiing. I have been told by FAA counsel that this is because in their estimation, a baby flying through the air and falling is going to sustain less injury than a baby who is body slammed by their parent into the seat in front of them. Don't kill the messenger, that's what I was told. I do agree though that it's up to each individual flight attendant to enforce the rule and if you think you're better off with your DC in a sling for those times, there's not a whole lot stopping you.
I only use a ring sling. After so many trips, it's really the only thing that is versatile enough to fit the majority of situations. I cannot tie a MT on one handed, I have to have a place to lay the baby down to tie the straps at my waist and that's not always convenient. I used to carry an onbu b/c of that, but we only take redeyes, so there wasn't a lot of need for it b/c I don't like tall bodied ones and all mine don't offer much head support for a sleeping baby. And, it's much harder on and off through security since it has to be taken off b/c of the metal rings. The ring sling also has to be taken off, but I can get it back on in a jiffy while running after DS (3). The baby can lie down if she wants in it, she can sit up, she can sleep, she can look around. I can use it for a blanket or a coverup - it's really very useful. And folds up very compactly if I don't use it (although I don't think that's ever happened
), whereas a MT, onbu and wrap are always so bulky, IME. I'm not a pouch gal, so I guess a pouch could serve all the same purposes, but I need a different size pouch to carry DD and DS, so that makes it out too. I know, a pouch is supposed to fit the mama, but I need a size smaller for DS or he isn't snug and secure in it. So...after all that, I would recommend a ring sling.
Have a fun trip!