I know both songs are beloved, but Scar is clearly evil so Be Prepared fits. However, Hukana Matata actually tells people to live in an apathetic lifestyle where you ignore problems. That's actually one of the biggest problems in the world: apathy. And, indeed, the Hukana Matata lifestyle nearly made Simba shirk his duty of going back to challenge Scar and it was only the arrival of ghost Mufasa that caused him to go back.
The sad thing is, it may have helped Simba in the short term after the shock of exile and the death of Mufasa, but, long term, the lifestyle harmed him.
Thus, I think they should remove Hukana Mattata and put back in Be Prepared.