Hi! I personally recommend +ninjutsu and -resistance. The reason is due to the ninja's personal growth. The Attack and Ninjutsu are very similar but Ninjutsu growth is much higher as shown below:

This stat is the potential of increasing Ninjutsu when leveling up. The reason I would go negative resistance over any other stat is a personal one. Life is a stat that helps with your ninja's ability to survive the next attack, the more the better. Taijutsu plays an important role in your damage, so I personally don't think it would be good to reduce it. Which means it comes down to Defense or Resistance, the age old question. To be honest, it is a personal choice, it does not make too much of a difference. However, the reason I decide to reduce resistance over defense is because I believe that standard attacks happen usually more frequently than mystery. Since standard attacks are usually Taijutsu, I chose to reduce resistance instead.

Note: This is something a wise person explained to me when I was starting the game a long time ago and I believed his reasoning to make sense. I chose to follow it ever since xD Hope it helps =p