There is some disagreement as to which fish is easier and some swear the nigricans is a stronger fish and others swear the japonicus is hardier. I've never kept either but like most tangs from the genus that I have kept they need lots of room (which you have) and clean and well oxygenated water and with a good mix of food most species are not terribly hard to keep. The Japonicus seems much more common in the trade and I would look for a fish that has been collected from a better place as the main factor.