In the year 1939 an unknown enemy called the Neuroi appeared. Their purpose is unclear and origin is unknown, but they attack and destroy any signs of human civilization they come across. Because conventional weapons are useless against them, humanity developed a striker unit known as the Strike Witches to combat the Neuroi. Miyafuji Yoshika is a girl with exceptional healing power but lacks control; she has been scouted to join the Strike Witches and fight the Neuroi, but all she wants to do is take over her family clinic. After witnessing an attack by the Neuroi on her way to find out if her father is still alive, Miyafuji resolves herself to fight for and protect the people around her. Who or what are the Neuroi and what do they want?
In the year 2079 CE, malevolent creatures known as Worms arrived in our galaxy, wiping out mankind quickly and efficiently. Finally, when the war's outcome seemed to favor the invaders, humanity united and nuked its own planet in order to survive. Though the Worms were defeated, 50% of the Earth's land mass submerged under the ocean, and 90% of all young men were killed in battle. Now, several years later, the Worms have returned. The world's best hope for survival is the genius Karen, the swordswoman Otoha and quick-thinking Eika; together, they make up the Sonic Diver Unit, a mechanized unit capable of taking down the Worms! Yet the powers that be are skeptical of its force and aptitude. Can the girls prove their worth, and take down the enemy?
Both titles (Sky Girls & Strike Witches) are creations of Shimada Fumikane and began as illustrated column in the magazine Comp-Ace about young girls as World War weaponry. Shimada Fumikane worked on both titles as character designer but only in Strike Witches is he being credited as “original work” and Strike Witches is also more closer to his original intend than Sky Girls was.
Both anime series are women-military based, even tho there are not many (if there even are) fights on Sora no Woto. The characters have lots of similarities, without the ecchi part on Strike Witches, and everyone learns to work together even when there are internal problems.
You could summarize with this formula:
"Sora no Woto = Strike Witches + K-ON! Art and Moe - (Magic + Ecchi)"
or
"Strike Witches = Sora no Woto + Magic + Ecchi - (Music + Moe)"