How do I tell an old friend that my new friendship isn't replacing her?

When I mentioned having dinner with Leila, my oldest friend said, "So she's your new best friend?" I laughed and said no. The next time, she asked whether I had told Leila something before telling her. I changed the subject.

Leila lives on my street. We started walking to the market together and now sometimes eat dinner afterwards. My old friend lives across town. We still see each other twice a month, and neither of us has cancelled those plans because of Leila.

I understand why a new name appearing in every other story might feel strange. I probably have talked about Leila a lot. But I don't want to start rationing what I say or reporting the order in which I tell people news. I value different things about both friendships.

Would you reassure an old friend each time this comes up, or say directly that you don't want to compare them? I want her to hear that she matters without giving her a ranking she can keep checking.