Well, I actually like both of them.
I don't like either of their designs. Kion looks like they took cub Simba and threw really random traits that weren't necessary on him - I don't know how to put it, but he kinda looks like a kid designed him, heh. As for Kopa... pass me the magnifying glass so I can see how different he is from cub Simba. Sure, there are some small things like smaller eyes and duller fur, but those:
a) could be the drawing style of the person who drew him in 6NA, and not actual, intentional differences
b) vary from image to image in the books
c) are hard to place because Simba and Nala look off is some pictures too
Personality? Kion wins this one. He is very nicely thought-out, his character deals with some important problems, and I can sympathize with him and how he feels. Still, if he says another 'Hevi kabisa'... my head is gonna pop.
As for Kopa, I believe people aren't judging him too fairly on either side, but that's just how I feel.
Some of his fans often look at fan-art and fanfictions, or form a brand new personality and look for him themselves - all while forgetting that they are changing a character that already exists. It's like 'Kopa' has become a default name for Simba's male cub, and not someone's interpretation of it. Kopa may not be Disney's, but he has his creator and artist, and everyone should keep that in mind when they alter his design and personality to suit the needs of their fan-canon. Same could be said for Ahadi, Uru, Mohatu Tanabi... If you are gonna invest your time into creating a brand now character, you might as well think of a new name for him.
And then come people who dislike him. Now, it's your right to feel that way, but please don't forget that the type of media Kopa appears in doesn't allow him to have much character development. He is given a few books, two audio-books, and that's it. He doesn't grow in that period, nor does he face anything really complex like Kion does, so it is a bit unfair to expect him to grow as a character. He instead faces small problems, and learns small lessons. The way TLK universe is interpenetrated in 6NA is a bit more realistic and less climatic too.
And I don't think he's a complete rip-off of his dad as a character - as someone here said, he's a whole lot brattier, snarky, and kinda mean-spirited when you think about it. He also openly confronts his father's ideals that lions should rule, and that hyenas are all evil, and befriends a hyena, which is something Kion doesn't do [yet].
But all in all, Kion still wins for me, because he brought back TLK in a way. With TLG, we are in for another ride in the universe we all love after so many years. We get to see characters we thought we would never see again, and that makes me happy.
Kopa has been kinda ruined to me by to me of his rabid and unreasonable fans, but I'm still his fan.
...
What if Kopa is Kion from different universe?