Thanks, all, for your thoughts!

If the upper height limit is 40 inches, I'm not sure we'll get 2 years out of it, but I still think it's worthwhile for us. And I had forgotten about the RiderSafe; but turns out it has a lower limit of 3 years AND at least 30lb, so we'll need to wait on that (but it will solve our cab-to-the-airport problem!)

And yes, DC to Buffalo is a short hop! But it's nice to "practice" with something like this on a shorter flight before going on a longer trip.