Let's compare both. 1.Supreme Kai : Supreme Kai, who watches the whole universe, isn't that strong compared to even mortals. In Buu saga, he is afraid of Dabura, who himself is a bit more powerful than SSJ Gohan. read more
But the catch is if Beerus or any God of Destruction for that matter kills the Supreme Kai of that universe, he too will die. They are bound to each other. Thus, even if Beerus defeats Supreme Kai, he will eventually die. read more