Answer:
it depends
Step-by-step explanation:
ellen degeneres obviously has a larger platform and has a bigger target audience. she is a lesbian and that can pose interesting topics on fame, how she has effected the world's views of homosexuality and how the world reacted when she came out.
Emma Watson is a feminist and known to lots of people as Hermione Granger. She has since left the spotlight with acting and has focused on activism. this can be interesting to talk about as well because of how everyone takes it, how opinions of her have changed and how she has effected feminism and how people view that.
if it's not about any of what I have talked about then probably go for Ellen because she has a larger platform.