question

You don't have to choose one over the other to implement a reportserver, PowerBI reports are hosted on the same reportserver as your SSRS reports. Then it's up to your developer team to decide when to use one report type over another.

PowerBI reports allow for interactive filtering, mouse-over and drilldown actions by the user, predefined in the report groupings. (helps users find the next question to ask, which is often important). PowerBI allows for easy use of out-of-the-box visualizations, and if you want custom ones, ties directly to R scripts to power the visualizations found there, so quite extendable.

However, unless you are publishing to the powerBI service, there are refresh difficulties.

SSRS reports are more static, you can create drillthrough actions by sending to another report that provides more detail on mouse-click, but it has to run that sub-report when you do so, not as easy to bounce from one to the other. But real-time refresh from your data sources.

I'm not 100% sure about subscriptions in powerBI yet, I think they're available, while I know SSRS subscriptions are available.