I've not tried the Palo Alto Sheraton yet, but I've been staying at the Westin frequently as of late. I've found the staff to be polite, the facilities clean and the gym more than adequate.
The Sheraton looks like a better deal for you from what you say, but then I'm not sure what's the most important thing to you. I tend to veer towards Westins in large part because they offer cleaner rooms, better gyms, quieter neighbors and better room service when I need it.
Also, with the rain here lately, the underground garage at the Westin has been useful.