Whether it is commerce cloud, EPIserver or other systems - there is a mandate for both, and I personally know of a couple of companies operating both Commerce and Marketing Cloud.
The main deciding point is to clarify which cloud will drive predictions of products - CC or MC, as this is where they overlap.
MC will drive any communication sent to customer. Landing pages can be in both systems, or one, depending on requirements, tracking etc.
CC will drive shopping / commerce experience.
Overall, I believe the key in this process is to ensure that you have the proper architecture in place, and the proper governance. Some of the questions that needs to be answered, among many, are:
- Do we have a tag manager in place? (to push collects scripts through) or is it a direct implementation?
- Which systems governs what experiences?
Marketing Cloud will, hands down, have the best tracking of the two systems - given that they can capture all the behavioural data from email, clicks, browse, sms clicks, apps etc. - meaning that over time, Marketing Cloud Einstein will probably be the rockstar of personal recommendations.
However, Commerce Cloud might be capable of better controlling page structure, user experience etc.
In essence, it all comes down to mapping, architecture and governance. What you would get, down the line, from running both would be a tighter integration and probably more direct synergies, which would be really interesting over time.
At present, I would advise to reach out to Salesforce directly, to hear what their vision for the split and relevance of the two products are.