In the scenario that the poster got rejected from both, there would be regret to have not possibly increased their chances by EDing at Williams.
However, what if the poster actually got accepted at both and had ED’ed at Williams so now must turn down MIT? That actually seems like the dilemma holding the poster back. If Williams was such a clear preference, the poster wouldn’t be asking about EDing to Williams but it’s the possibility of getting into MIT and having to turn the school down that prompted the post in the first place. Personally, I do not like ED for this reason among others. It pressures students into making a firm commitment when it would be to their benefit to see what options are available to them and at what price. In my view, EDing is giving the students the short end of the stick for a boost of some sort in admissions.
In this case, I would suggest the opposite. Do not ED because if you do, you might always regret not being able to compare the choices. As someone else suggested, there may not even be an ED bump for an unhooked candidate so why remove one’s choices because if you actually got into MIT and then got into Williams during regular decision, you can compare the two schools and visit both during their campus preview weekend. If you got into neither, you may regret not having EDed to Williams but you will actually not be sure that if you had, the outcome would have been different and at least you gave yourself the possibility of having options which EDing to Williams might take away. Another thing to consider is that between now and May, another school might come into focus that provides good merit or you find out more about the program and actually prefer it such as a school from the list @dad9000 provided which you would consider in the spring but which may not even be an opportunity for you to explore if you ED to Williams.