Shadow Vs fiery: I used to always use shadow, but after fiery got buffed it is so useful at clearing Tartarus quicker, especially if you have rocket bomb when using Eris.

Chthonic Vs dark: I honestly see no reason to use DR, as it means there is no consistent way to heal in Elysium/Styx, and the extra health at the end of each biome is overkill.

Death Vs stubborn: I have the same strategy as you, DD for speedrunning/casual play, SD for high heat (32+)

Greater Vs ruthless: I used to use GR for the longest time, but on casual runs I pretty much always take RR. 50% dodge chance is really strong, and unless you are speedrunning and you need the extra movement, only having 1 dash isn't that bad imo. However, if I'm speedrunning base shield (zag/chaos/zeus), I will take ruthless reflex because if you attack after you dash, you will carry your momentum forward and slide along the ground, which ends up being faster than dashing twice.

Boiling Vs abyssal: I have the same reasoning, more damage is better for survivability.