I like the Marco Polo over the Crown Plaza because the area is a bit more subdued than the Wangfujin area the Crown is part of. If you're after late night markets and crowds then go for the Crown Plaza. Both are OK hotels in your price range.

The Crown is on the east of Tiananmen and the Marco Polo is on the west...both about equal distance from the square (this distance can be walked). Also both have subway stops in their vicinity. By taking the subway from the Marco Polo you can easily access the entire Wangfujin area that the Crown Plaza is a part of without actually staying in the area.

Either way you'll be well positioned the enjoy the many fascinating sights of Beijing. Enjoy your trip.