anyone ever been to either Westin Seaport Boston or Aloft Seaport Boston?
Stay at the W for 20k and enjoy a beautiful suite and breakfast courtesy of your diamond status. The lobby has a lovely and lively bar with lots of people walking in till midnight.
Now that someone responded to this again I figure I'll jump back in with a little more info. Sounds like your original question was which of the Cat 5 - 12k/night SPG hotels you should stay in. So I'm assuming the aformentioned W (and also the Liberty) is off the table at Cat 6 - 20k/night. I would still suggest at least bumping up one notch to the Sheraton at 16k/night. It's also a category 5, but I don't really know how they determine when it's 16k and when it's 12k. However, the difference in convenience of location can't be understated.
The 3 hotels at 12k are way out of the way - renting a car would be a very expensive proposition, so proximity to attractions and public transportation is very important. The Sheraton (and really any of the hotels in the Copley Square/Convention Center area) is perfectly situated. It's walking distance to Fenway, short rides via the T to the Sam Adams brewery, minyan and kosher food in Brookline, Milk St in the Financial district, and much more.