Avelin's cute with her hair shorter, but Eiren is so cool, and seems to really treat the player as a friend and companion that's helped her. I'm cheering for Eiren all the way~
Eirawen seems to be into the player as more than just a friend. There's a few dialog exchanges that teases her 'attraction' to the player regardless of their gender. If anything she'd rather you just work for her in the kingdom loafing around in the royal palace if only just to give her a sense of security, basically a living teddy bear all things considered.
Avelin on the other hand sees you on a more professional light. A reliable and powerful ally that she trusts with more sensitive information. The relationship is also much more professional. From her point of view, the player is a specialist operative of the organization whose powers, experience, and expertise are invaluable in combating the current threat they are facing and is an individual of similar rank. (She doesn't treat you as a subordinate, and is much closer to being a peer).
On that note I prefer Eirawen if only because she has a much more personal connection to the player. She also has a much better character arc of which we were an active participant in. We meet her as a highborn princess who had the world at her beck and call, we then stand witness as she loses everything, from her friends, to her father figure and at her lowest was just a sobbing mess that was moments away from being murdered. She ends her story as the wise Queen of Tara whom the player helped build directly, using their status as a hero they built an alliance of various factions and installed her as the leader before securing her position by helping her overthrow an unfit ruler. By comparison, Avelin begins as a well respected member of the Alban Knights, and ends pretty much the same way, her only change in character is her coming to terms of her brother's death.
did you misread eiren as eirawen, or did you really want to compare avelin with eirawen? lol