Different types of gates use different amounts of silicon area. Combining gates "cancel out" different number of transistors. So the gate density you'll end up

Different types of gates use different amounts of silicon area. Combining gates "cancel out" different number of transistors. So the gate density you'll end up with also depends on your specific application. I think the number of transitors per unit area would be better. Though advanced applications mess with transistor sizes too, the situation would be better than gates.