Interface Design: Mediator vs UM

hello experts,

there are certain card related services available in certain IS. The services are available for others to use via mediator too. I can also publish the canonical to UM to get the card related functionality invoked.

In this case, what is the best way to invoke, this is an EAI ( A-2-A) integration scenario.

Happy to share other info if needed.

Regards
Sasa