Mahoutsukai ni Taisetsu na Koto

Both are slice-of-life series, with something fictional and fantastical (magic in Someday's Dreamers, and the Haibane in Haibane Renmei) existing in a mostly "regular" world. Both series have a relaxing, calm atmosphere, and the female protagonists of both have fairly similar personalities and demeanour. They also have a vaguely similar art style. 

Both feature female leads trying to resolve issues and both share mostly procedual formats in it too and both can be pretty cathartic to watch too. 

Both slow paced slice of life's about girls going to a new place to do training in the profession they plan to pursue in whilst making encounters with new people on the way to grow as people. Main difference is that there's a bit more drama in Mahoutsukai as well as there being magic used whereas Aria is more fluffy but has no magic at hand. But apart from this, some of the mysterious and somewhat otherworldly encounters both have at times within the normal everyday are quite a similar aspect too.