Hi Yoshitada Kaneko san,
I completely agree with Big Choi. Costing Based is a better choice.
However, i have recently read some blogs on Smart(Simple) Finance wherein the focus is shifting from Costing Based COPA to Accounts Based COPA.
Preview of New Smart Accounting Part 1
Preview of Smart Accounting Part 2 - FI-CO is Changing Reporting
Please have a look and incase there are chances of your client migrating from Normal ECC to Smart Financials in the near future, it would be a wise option,in my opinion, to activate both types of COPA.
Best Regards
Surya