What does the subthreshold swing of a mosfet actually mean?
I'll give it a shot. The subthreshold swing definition describes an exponential behavior of the current as a function of voltage. The sign of the parameter should depend on the type of the device (n- or p-), so it's easier to just say that some voltage difference is required for a decade of change in current. Except for the sign it's the same number up or down (if +50mV gives 10 times more current, -50mV should give ten times less). As for the question which device is better... that depends. If you need very low threshold voltages, you don't get a choice, there will be leakage. If you need high operating voltages, then you need thick oxides, which means little leakage and the effect doesn't play much of a role. That used to be the case for most mosfets in the past, but at the current integration density we don't get the next to ideal devices of the past any longer, whether we like that, or not.