Tatham Oddie prepared a table outlining the differences between Virtual Earth platform and the MapPoint Web Service --

Map Styles
Virtual Earth: Road, Aerial, Birds eye
MapPoint Web Service: Over 30 different styles (optimised schemes for night viewing, etc) however no aerial imagery
Integration style
Virtual Earth: JS control (best for embedding in web pages)
MapPoint Web Service: SOAP web service (usable anywhere)

View the full summary --
What’s the difference between Virtual Earth and MapPoint Web Service?