Everywhere has it's good and bad points, and it does greatly depend on when you are traveling. Out of the Tsavos, I would always go East. I would only bother with the Mara for the Migration, but having been there many times, that is just me! I hate to muddy the waters even more, but Amboseli is also worth considering!
Hate to sound picky, but that depends on which Lodge you are in, you may well land very near your Lodge?! But, if you don't, they are never that far away, and you can always spot things on the journey to and from the airhead, keep the camera and binos handy, and think of it as a practice game drive!
Right, I am off to pack, as I depart for NBO on Monday, whichever you choose I am certain you will enjoy it, have a great time.