I would not want to walk from Harvard to MIT - they are at opposite ends of Cambridge! There is great public transport. And transport from Logan. Cheapest hotels would be in Central Sq area between the two. Finding parking places is difficult and car not necessary. If you rent a car bring lots of quarters and plan extra 15 min to rid around looking for spot.
Wish you all good luck
:D :D :D :D :D :D :D :D