Zachriel,
I thought an Airport could only receive one transit a turn, as well as only send once ... are you saying you'll send from multiple airports to a single receiving destination, on a single turn? If an airport can receive from more then one "transmitting" airport, on a single turn, then you are absolutely right, airports would be worth the extra cost to build over harbors ...
PLEASE confirm this, or lay to rest, I was very sure an airport could only receive ONCE, after the first unit, no more to that airport on THAT turn ...
Dang, one can definitely learn new tricks ... this IS a complex game ... If the above is true, my late history invasions just got MUCH deadlier ...