The W is a MUCH nicer hotel then the Westin CC. The Westin is very old, in need of a renovation, and almost all rooms face the interior. I would say it is actually the crappiest Westin I have stayed in. Age out of the way, it was a perfectly clean and safe room. If you just need a cheap place to stay and will be out exploring the town, you will be fine at the Westin. If you want to have a nice escape though, it might be worth sticking with the W.
The area around the Westin is VERY dead, but as another poster mentioned you can walk to several more lively neighborhoods in about 15 minutes. Head up 14th st to Logan Circle at P st or the U st area (guess where that is?). You can also head west on M st and will start hitting the Farragut / dupont area at 18th and M.
The W does actually have more going on the neighborhood around it now (such as Hamilton across the street) in addition to being very close to the mall. As the other poster said, Chinatown (or Logan Circle mentioned above) is your best bet for neighborhoods within walking distance of the W.
Oh, as far as metro access, Metro Center (one of the main stations) is only a block or two from the W. The Westin CC is closer to 5-7 minutes from McPherson Square.