Both Anime are spin-offs of harem/action anime which was released in a past season. The main similarity with these two spin-offs is that the story now follows a newly added main female character and her interactions with one of the main girls from the original series.
There are many things that are the same about both of these anime.
First off both stories have a character base of mainly females, students too.
Second, the main character is a weak girl who might have some sort of twist about her.
Third, there is a strong protagonist who is understanding and willing to protect the main character.
Finally the plot seems the same in my opinion, both take place at a school that is not the ordinary, they both have something to do with fighting, and there will most likely be a twist that changes your whole perspective of the anime
The main character from both series want to be like their senpai. Akari to Aria and Tsugumi to Maka. Both taking place in a school setting. Soul Eater NOT has supernatural events while Aria AA does not. There is something special about the main character. Aria and Maka see's potential in Akari and Tsugumi. Akari/Tsugumi makes friends who help her out.m The anime is a side/spin off of the original manga. The original characters make an appearance to help out once in a while.
Taking place in the same city but involving a new character. Hidan no Aria original characters are still present but are not the main focus. Including the main male protagonist. Same with Railgun which focuses on the other characters but not the main male protagonist. One involves science and magic (Sometimes) While other does not have any of that. Both are very action packed. There parent story was based off from a light novel. Then later on had a manga that was a spin/side story of the original light novel series.