The most difficult alternative

For me, the most difficult choice is between Genies and Nagas.
Genies are definitely stronger, but they are more difficult to build and are more of a late-game unit.
Nagas are cheap and tempo-based; if the map is small, I choose them, and I can start fighting very actively right away.

Elf and White Tiger.
If I can't build fast Griffins, I choose the Tiger because it's tempo-based and fast, and I can carry it in the army. But if I can build fast Griffins, I build defensive Elves, and the Griffin + First Strike ability provides experience gain.

And the only choice I haven't 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.

Are Genies more difficult to build? Are Nagas cheap and tempo-based? - This is only true in Equilibrium. In the original Heroes IV, there's no question.
If you plan to build Thunderbirds, then Harpies are naturally needed. Besides, sometimes they are more useful than Nomads. Nomads are cheaper to build, which allows you to build Beetles and Behemoths faster. Neither Harpies nor Nomads will play a decisive role in battle.
But here's what I've noticed. With similar parameters for Tigers and Nomads (Tigers are even slightly weaker), Nomads somehow die too quickly in battle.
But usually, the main criterion for choosing is the speed (and, accordingly, the cheapness) of building a powerful army, i.e., level 3 beasts.