Just for the record, since we seem to have a fair number of new posters here, the FAA's recommendation (and mine, FWIW) is to use a carseat if possible.
On another point that has been mentioned re: FA's telling you to do something that might seem to you to make no sense. The brace position has changed, true, so that argument is no longer an issue. However, if for some reason you do get caught in a difference of opinion with an FA -- do not argue and especially, DO NOT ATTEMPT TO DEFY THE FA's REQUEST! It isn't really a request, it's an order, and if you do not immediately do as you are told, without argument, you risk getting charged with a felony. (The charge is "Interference with Flight Crew Members or Attendants" 49 U.S.C. § 46501(2))
Since 9/11, flight attendants do not tolerate *any* opposition or any behaviour they interpret as threatening or defiant, and will press charges at the drop of a hat. While I think civil disobedience has its place, an air trip with a baby in tow is not it. They WILL manhandle you to assure your compliance if necessary, and if that is necessary, you will be met at the gate by law enforcement, and your baby will not be going with you to the jailhouse. (We won't even discuss the possible reaction of fellow passengers.)
If you know that you are in the right, AND have up-to-date documentation to prove it, you have a case and can present your documentation politely to the lead FA. However, if the lead FA still disagrees with your POV, you need to drop the matter until you are off the plane and can file a formal complaint with the airline.
FTR, TSA will react much the same way to any kind of argument, though they don't have as much power to make your life h**l. You want to leave all the mama-bear behaviour on the curb outside the airport -- it will get you in way more trouble than you can imagine, which is not a good thing for your kids.