I think there are certain traits in both that are endearing and also annoying.
Ginny: I like her because she seems a little like Lily from what Rowling had written about Harry's mother's character. Rowling said that Ginny was the smartest in her year (like Lily), she was brave and extremely loyal to those she loved, and she hated Dark Arts and anything to do with it. She was one of those staunch Dumbledore supporters who never got tempted to join Voldemort's side, and I liked her character ever since the 2nd book when she defended Harry against Malfoy in Flourish and Blotts. And in the end of Chamber of Secrets, when Harry saved her from Tom Riddle, I just kind of knew she and Harry would end up together.
The qualities I don't like in Ginny are her recklessness, and the way she sometimes acted before she thought. It helped everyone sometimes, but also at other times, got them in trouble. Also, the ways she nagged at times got annoying.
Luna: From what we know of her, she's an extremely loyal person who values the friendships she made during Order of the Phoenix. Smart and often misunderstood, she seems like an innocent girl who can never say a bad word about anyone. She also has a calm air about her and doesn't go into panic attacks when faced with a critical moment. These are the qualities I find endearing in her.
The qualities I don't like about Luna are her extreme naivete and the way she keeps too calm even during battles. Especially in the battles during the 5th and end of 7th book, people had to save her a few times because she wasn't paying attention to what was happening around her. Sometimes, it seemed like Luna had a lack of concentration.
I like both characters, though I felt they were not developed enough. Both Luna and Ginny were at times completely different from each other, but similar in another way when it came to ideals.
But there's something about Ginny that I like better. I also like Luna's character a lot, but Ginny more.