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Elsie will clearly cause more trouble in the long run than Kirino. Think about it, with Kirino you just need to get through to her dere side. It might be tough, but you can't call yourself a true aniki if you're not up for the challenge. With Elsie, however, no matter how much you love each other, she will keep messing everything up and being a huge bother. Imagine when she becomes your housewife and insists on cooking every meal for you, so for breakfast, lunch and dinner every day you have to go through the challenge of not committing suicide by eating her food while not hurting her feelings by not eating it.
Also, Elsie thinks your otaku hobbies are unnatural and that you should stop playing games all the time, while Kirino would encourage you.