That's a great question. There are some other different variables to consider in order to determine what is the best course of action for you.
First, if you put down 20% now will that empty your nest egg? The reason I ask is that suppose once you move into your new home something comes up that needs repairing. Will you have the money to get the work done? If not than it may be better to go with PMI. It might add $100 to your monthly payment but that way you won't be strapped for cash if something happens. Something to be aware of is that banks are supposed to remove PMI automatically once you have reached 22% equity in your home. You will have to pay close attention to that because banks rarely do this without some pushing by the owner,
The other thing to consider is that prices in the in Cambridge are going up. If you wait you will most likely be paying more for a home as well as higher interest rates. If you buy now and put 15% down you can always make extra payments to the principal of your loan without penalty. This way you will have money in case of an emergency and perhaps a little more peace of mind!
Hope this helps.