Molly our adopted child is 11 and was 9 when ds, 3, came to live with us.
The first year was a real mixture of wonderful and awful! DD (11) was very jealous and DS (3) was very clingly. After the first year things evened out and have got a lot calmer.
My children love each other, a lot, they also fight and argue just like any siblings.
I cannot quantify the love I have for my kids, I love them both fiercely and deeply. I think the differences are that known my dd now for 11 years and my ds for only 2.
My dd has her own needs, she's not on the spectrum but has autistic tenancies, and this gives her vulnerabilities etc, which I've tuned into. Where as my son is very bright and 'normal' but emotional. He also has certain needs and I think my love for them both is very much tuned into who they are.
In reality I do not have favourites and try very much to be fair to them both, which I know is very important for kids.