It depends on what you want. The Westin is by far the nicer of the two. They regularly upgrade to Executive Rooms which are fairly spacious. The regular rooms are on the smaller side. Forget about a suite upgrade though. The hotel only has a few (it used to be a DoubleTree) which are almost always booked for cash. I would say that the Westin Pasadena is the 2nd nicest hotel in the area, surpassed only by the Ritz-Carlton.
The Sheraton always felt gritty to me. The chances of of upgrade are much better and there is a lounge. It used to be a Holiday Inn. The regular rooms are tiny, but unless the hotel is sold out they will usually try to give you a reasonable sized room.