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I have both the 5D2 and the X100S.
X 100S advantages: small, lightweight, excellent fill flash, discrete, good optical finder, nice direct aperture control.
Canon 5D2 + 35/1.4 advantages: Faster, better IQ, better ergonomically, great optical finder; for me , at least, much more intuitive controls - especially ISO control (I don't like the auto ISO much on the Fuji), great battery life, excellent shallow focus on the 1.4, great bokeh.
X100S cons: seems like a gadget, a little fiddly to use (changing ISO), less depth of field control, poor framing, utterly crap battery life.
Canon 5D2 cons: weight/ bulk, bulk/weight, bulk, bulk and weight. No easy fill flash.
Better than either, for me, is the Canon 5D2 + Voigtlander 40/2 with the Maxwell screen. Manual focus heaven. It's still too heavy and bulky, but the direct feel and control makes it better for most of what I do.