It would appear that SPB took a lot of elements from HTC's Sense UI to create their shell. The main difference I would imagine (having not used a WinMo device myself) is that the mobile shell is exactly that, a shell. It is more like an app that installed rather than being apart of the actual operating system like Sense UI is. This could be a plus or a minus really. With the mobile shell you could always turn it off if you wanted where as Sense UI cannot.