Of the 3 nightingale powers, I'd say Shadowcloak is potentially most useful. something like Subterfuge can be done with a dual cast of frenzy(illusion perks and free cast gear are helpful). Strife isn't that great if you have good armor and weapons. Shadowcloak can let you go invisible many, many times during those 2 minutes by simply standing up and crouching again. Unlike the spell, you won't need to worry about the light of the spell in your hand giving you away if you don't sheathe it or swap to something else right away(see vampires that try to run away from you when low on health). The best part is that if you are still invisible at the end of 2 minutes, you can choose to remain that way indefinitely as long as you don't interact with anything.
Shadow warrior can be very OP. If you have assassin's blade and shrouded gloves or hand wraps(available whether you join the Dark Brotherhood or choose to wipe them out), you can land 30x sneak attacks right in an enemy's face just by crouching in front of them(daggers are better for this due to their speed and silence). This doesn't work against all enemies, though. It seems that once undead detect you(draugr, vampires, etc) you can't get hidden again so easy. Maybe it comes down to a draugr's high detection levels or vampire's generally high sneak skill.
You can choose to have both. I play on legendary difficulty and power level sneak in Helgen(and 1H) so those sneak perks and damage bonus help counteract the extreme danger that may be posed by early game encounters until I can max out my crafting skills. On the other hand, I recently used Shadow Warrior(in conjunction with a highly smithed dagger and full set of sneak gear) to one shot Karstag from the Dragonborn DLC(considered one of the toughest enemies in Legendary Edition or the remastered Special Edition alongside Ebony Warrior).