For me it's about what user interface paradigm you are modelling with the click - is it select (think mouse single / double-click) or is it options (think mouse right click).

For me as there isn't a single action I always want to perform when selecting an episode - I need a context menu to present options to me.

Can't remember the last time that I even read any shownotes. I would also like to see the options available extended in the pop up as well (e.g. mark episode as played etc).

Displaying the show notes limits the number of options that can be presented - which is likely in turn to push extended options into a menu on this new stacked window. Before you know it you've brought one action one click closer and pushed several others one click further away.

I haven't tested it but I would expect that the context menu will pop-up quicker than a new stackable window with shownotes would.

Just my opinion of course.