The external astragal bars will match the profile of your window, they’re surface mounted and the glass unit will have internal back to back spacer bars, using this option will most authentically replicate traditional timber windows which had multiple glass panes. For example Georgian or Victorian style.
The internal fret will also create the look of separate panes, albeit less authentically. The benefit of fret over astragal is that the windows are easier to clean without having to clean around the surface mounted bars. Notwithstanding that, the astragal bars are still the most popular and attractive choice.