The external astragal bars will match the profile of your window, they’re mounted onto the surface of the glass and within the glass unit you will find internal back-to-back spacer bars. Opt for external astragal bars if you require the most authentic solution replicating original timber windows with multiple glass panes e.g. Georgian or Victorian style.
The internal fret will also create the appearance of separate panes, albeit less authentically. Internal fret makes it slightly easier to clean the windows due to the bars being inside the glass unit.