A.S.
We've traveled with both the travel system (when kids were in infant seats) and the Maclaren Triumph and, recently, our Kolcraft double umbrella. All equally easy to get through security, drop at the gate, etc. Traveling alone I did find the Maclaren to be great b/c it's lightweight so I could get it on the belt to go through the xray machine while still wrangling my then 17-month old. We traveled with both my girls when they were 7 months old and the easiest way I found to deal with them was wear them (in a Mei Tai or Bjorn or what have you) and put the stuff in the stroller.
One thing we did learn on our latest flight is if you don't purchase a ticket for a lap child, still gate check the carseat (as opposed to dropping it at the desk by the gate if they encourage you to do that) because they pretty much always say the flight is full at the desk. Both ways on our latest flight it in fact was not full (contrary to what we were told at check-in and at the gate) and the flight attendants grabbed our carseat for us and helped us rearrange with other passengers to get our infant in an available seat. We were VERY happy to have done this on our flight home as we flew through a terrible storm and actually missed our landing and had to take off again into a terrible storm and then make an emergency landing at another airport. Couldn't imagine having held my 11 month old through that!
We did find flying at 7 months really easy with both our girls. They both just slept the entire flights both ways.
Good luck!