We have stayed at both properties many times and have no complaints.

Your post mentions the cost of parking so that leads me to assume you are going to have a car. The Cambridge property is more isolated. Whether this is good or bad depends on what you are looking for. If you are going to be using your car during your stay, the Cambridge location might be easier to get to and from (depending on what your plans are). If you are not going to use your car, the public transportation options and walkability of the Boston property are better.

In terms of parking, especially on weekends, there are much cheaper parking options than the $40 you reference. For the Cambridge location, there is free on-street parking after 6 p.m. Friday or Saturday depending on the parking spot on the street right behind the Hyatt - you just have to pay attention to the street signs. For the Boston location, there are much less expensive options than $40 a day. The bestparking website should give you a number of options to look at.

The rooms and general condition of the properties I would consider equal. The view from the restaurant at the Cambridge property over the river is very pleasant. There is no view from the restaurant at the Boston property.

You didn't ask, but the views from the Boston Harbor location are great if you have a Harbor view room but that property is quite isolated also and you are pretty much stuck with on-property parking unless you want to get creative (that could include parking at one of the MBTA lots, taking to the T to the airport stop and taking the water shuttle bus that stops right at the Hyatt).