They are both centered around the idea of humans fighting aliens of unknown origin in war machines. The atmosphere surrounding them are very similar as well, giving a vibe of loneliness and sadness. The main characters are both in a symbiotic relationship with their respective machines. While heavily focused on action, both anime pay significant attention to human relationships.
Both involve some kind of war between the human race and aliens. (In KnMYnB this story is more or less pushed off to the side )
Both are visually stunning pieces of anime that are a treat for the eyes.
Both of them are about war, dogfights and personal problems. The main difference lies in the fact that Yukikaze is SF and Simoun is more like Steampunk, but if you'll like one of them at least a bit, you'll most likely enjoy the other one. Both series also have a wonderfull Soundtracks, fitting the events in the air and on the ground. Simoun is also known as a yuri anime, but it's not really like most of the yuri series.