Powder Brown tang ??

[Aquattro]

Most people will tell you that the japonicus is the easier to keep. Almost all my research confirms this. My experience does not confirm this. But I can only compare 2 fish I have had, not the average. Dale D. from animal house states that neither are very easy to care for.
If you do a search on RC, you'll find lots of threads with experience keeping both of these fish. I think the secret is getting a healthy fish to begin with.
If you're planning a japonicus, I would stay with the plan. I just like to point out to some tang cops that no tang is "impossible" to keep. I have a nigricans because I was told it was the easier to keep based on Scott Michael's opinion, and so far it is doing well. I expect to report the same thing in the years to come.
Whichever you choose, make sure it is fat, healthy looking and eating well...good luck!!