Whichever commander you choose, the deck will be mostly nonfunctional, so go with your heart
Both put counters on creatures, invalidating their ability to attack you or planeswalkers you control, but Nils works more like a Ghostly Prison effect based on the number of counters creatures have
A deck based around this commander takes some focus, youll want to make sure you hit the right cards after each discover but can be really fun to design
And it's a bomb in a Bloomburrow draft, so you're always happy to see it
Under Satoru, Ninja Tribal is an aggressive low(ish) curve deck looking to sneak in your ninjas with the help of evasive 1-drops like Mist-Cloaked Herald and then keep them sneaky with evasion enablers like Aqueous Form or Wonder
Your opponents will likely spend their combat phases attacking each other, making your job easier