For my level of play a Nickle is better.
Even on a 12.5mm tip.
It is more forgiving when I need to hit the cue ball dead center with no side spin.
I am willing to give up a bit of spin for better accuracy.
Helps me make those straight in long shots.
they both give the same spin on a pool cue
you won't see a difference until the tip is under like 9-10mm
Intending to hit center of the cue ball but hitting slightly off center, therefore imparting spin to the cue ball, will be lessened with a flatter tip.