Depends on how the Darkness is defined. Images (Light Source) can cut through Darkness. Change Environment (Bright Light) can cut through Darkness. All depends on how the Darkness is defined.
Choosing one over the other, at some point a GM is going to step in and ask "why?" and vet your character. At 96 Charges/day I'd just tell him to buy it without.
Since ninja did assassinations and spying, there's not a lot of difference there.
Depends on if you're looking at history or Hollywood for your ninja inspirations.