Good choice with the Sheraton. I stayed at both this week and, after the 2K/stay "new hotel" bonus ends, I'll be at the Sheraton.
The words that come to mind with the Westin are "cramped" and "dark". I was on the 10 floor. After being joined by four people on 9 on my way down in the morning, we stopped at every floor, where people looked in saw a full elevator (yes, full with 5 people) and continued waiting. The hallways have low ceilings, medium brown walls, dark brown carpet, and nothing but wall-mounted lamps with screw-in lightbulbs for lighting.
Room service - soup and a pizza - took over an hour. The guy delivering said nothing until after I had signed the bill, only then saying "waiting a long time, huh?" and left. Half of the soup has spilled out on the tray and the pizza was some of the worst ever. A newspaper did show up in the morning - it must just be for the "SPG Preferred floors". No lounge and a very average bar downstairs. Most of the rooms also are adjacent to major roads, although double-pane glass and thick curtains made this mostly a non-issue. New beds were the only positive - standard excellent bed/linens/pillows/etc.
At the Sheraton, 2 days later - open, spacious, bright - room service in 20 minutes, a very nice gentleman, food was decent. The lounge was very nice, better than the Marriott where I spent the last 2 months (Megabonus - 57K points and 24 extra night credits...). The beds didn't quite match the Westin, but everything else was better.
I will be staying at the Sheraton for the next few months, possibly hopping over the Westin periodically as both have the 1,000 bonus points/stay starting in July.
Or at least I think I will. I found our corporate rate at the Ritz-Carlton, closer to my project site, is $10 more per night. I'll be trying that out on Monday....