Hi Piet
If you are on a green field project and COPA is a major ask from a business, i believe S/4 HANA Finance addresses major gaps which do not exist in AB COPA in ECC. Refer the link:
Account Based COPA - Simplification with Simple Finance
If however, you are on ECC, the data for AB COPA is stored in COEP, so it does have an impact on your other CO line item reports. However, basic functionalities like COGS break up and variance postings are not addressed in AB COPA and it makes sense to go in for costing based COPA from a functionality perspective.
From a sizing decision perspective, it does not make much of a difference whether you decide to go in for AB COPA or CB COPA. The sizing comes into play if you activate both in ECC.
Thanks & regards
Sanil Bhandari