I would guess the head gasket would leak coolant rather than oil on the outside since the coolant passages are closer to the outer edge. Theoretically, the oil could snake a path through the head gasket material w/o touching the coolant, but it is much more likely that the two would mix (as you mentioned) causing white smoke, loss of power, loss of coolant, etc. First thing I would do is see if you can exactly pinpoint where the leak is. If not, I would replace the valve cover gasket and go from there (Felpro gaskets seem to work fine IMO). A thin bead of RTV on either side of the valve gasket will help ensure your leak doesn't reappear.