Actually if you take an XOR gate with a naive translation into NAND gates you would use 20 transistors, but if you don't need a large fan out from the output yo

Actually if you take an XOR gate with a naive translation into NAND gates you would use 20 transistors, but if you don't need a large fan out from the output you can actually do it with 6 transistors using a pass gate layout (which is neither NAND nor NOR). So typically you don't use NAND vs NOR because of transistor count.