Kakashi and Tsunade both offer different things in terms of lessons and skills they can pass down to Sakura. It's really hard to compare them both when they offer different things. I think Sakura will miss Kakashi as a teacher but when she begins to work with Tsunade, she and Kakashi will probably be hanging out at that point anyway, able to shed the title of student and teacher.

We'll see about that gem, it had to be earned after all.