I've always preferred cub Kovu, that's when he's all innocent and stuff :3 Adult Kovu's personality wasn't developed enough, IMO. Well, not that he had a developed personality as a cub but I doubt he could have more screentime as a cub, whereas adult Kovu could. But of course, Disney wanted to finish the movie ASAP to get their bucks. XD
I always preferred Adult Kovu. As a cub, he is quite annoying. "Bet you're daddy's little girl!" Says Mommy's little boy. Grow up.
And he does that. Which is why I prefer adult Kovu. Actually, is it adult? I thought that in the end of Simba's Pride, Kiara and Kovu are in their late teenage years.
I prefer Cub Kovu because as an adult, he's emotionally scarred and whatnot, while as a cub you can see hs true personality since nothing has made him change.