I prefer Kiara as a cub, she's cuter that way :3 Plus, I don't support the Kiara/Kovu coupling when they're adults, which doesn't help. Also, I find it sad when all the lion's grow up, don't ask why xD I prefer 'em all as cubs.
I love Kiara anyway... But I prefer her as an adult. She still looks cute and innocent, but her wisdom starts to show up as well. Combining those make her awesome! And I like how she'll never learn to hunt
And I think I've decided on adult Kiara. But, that's only because of the last 10 minutes of SP, when she was wise enough to know what she needed to do, and brave enough to actually do it.