The most difficult alternative

For me, the most difficult choice is between Djinn and Naga. Djinn are definitely stronger, but they are more difficult to build and are more late-game units. Naga are cheap and tempo-based; if the map is small, I take them, and I can start fighting very actively right away. Elf and White Tiger. If I can't make fast Griffins, I choose the Tiger because it's tempo-based and fast, and I can carry it in my army. But if I can make fast Griffins, I build defensive Elves, and Griffin + First Strike provide experience gain. And the only choice I haven't quite decided on is Nomad/Harpy. I have an explanation for building all other units (opponent, map size, resources, units in the area), but not for these two.