Park Hyatt SF
I haven't stayed at the Grand Hyatt but the Park Hyatt is primarily a business hotel near the Embarcadero Center. It may be fairly dead around the hotel during Thanksgiving but the facilities themselves would make this a no-brainer for me. Contemporary decor and great service. Lobby has a small footprint and rooms might not be as large as what you might get at the Grand Hyatt.
It's a nice hotel, probably much nicer than the Grand Hyatt (definitely nicer than the Marriott).
There is a nearby BART station but you are probably 2/3 or 3/4 of a mile away from the Powell/Market street area you know. However, I would suggest checking out the Ferry Building which is a fairly short walk from the hotel.
If you like an area that has more of a touristy feel, you could try the Westin St. Francis which is right in the heart of Union Square or even venture up to Fisherman's Wharf.