I think it all boils down to how much you really know about building computers. If you know your stuff (be honest with yourself) then build it yourself as you can save money and spec the PC to your standard using quality components. If you have basic knowledge of system building then probably best to buy pre built as you get the warranty etc...

If you really know your stuff you can build a system that will out perform the XPS range for less money. It also depends on what your budget is.