The manga for The Voices of a Distant Star is an improvement over the film in some ways. The characters and their longing for each other is expressed more clearly in manga form and the bland character designs of the original have been improved on by Sahara's art. The shadows and shading give a sense of melancholy.
Whereas the original had virtually no dialogue and a cast of (as far as I remember) 2 characters, the manga expands on the basic plot quite a bit with new characters and conflicts. The overall character arcs of Noboru and Mikako end up resembling an alternate universe's 5 Centimeters Per Second, especially with the other love interest of Noboru whose role is that of Kanae's from 5 Centimers Per Second.
I also enjoyed the extra details in the ending compared to the film. In the film, I was left with a "That's it?" sort of feeling at the inconclusive ending but the manga for The Voices of a Distant Star perfectly wraps up the story of how our leads separated and will be reunited with each other. The only thing I could criticize about this adaptation is that it doesn't evoke the same awe from Makoto Shinkai's gorgeous scenery and beautiful mech fight scenes, but, of course, that is something unreproducible in manga format in my opinion.