The markdownpart package contains the actual code for rendering Markdown content in KDE apps (not just the in-editor syntax highlighting that already exists in Kate/KWrite), so yes that would be needed for the Markdown preview to function.
As far as which one, the markdownpart package is described as a KPart, for KDE apps, and the gedit-markdownpreview-git AUR package is described as a plugin for gedit (the GNOME text editor). The former would be made to function in support of KDE apps, the latter would be made to function in support of the specific gedit GNOME app.
Hope that helps,