The final “W” stands for WES- Windows Embedded Standard.
WES is the new name for XPe. MS marketing thought it would be good to have something new in this market sector as there is currently no Win7 embedded available – although in beta.
Currently there is no difference between XPe and WES, beside some additonal software like Silverlight, .NET 3.5, etc. But in the future MS will stop support for XPe while WES will last longer.
So new installs should run on WES.