When it comes to this, it boils down to character design. Who is more memorable and impressionable.
I’m going to give my argument for Cairne, and why he should come to the Nexus and not Baine.
Cairne has a very unique character design. He is senile, slow, and sometimes will doze off during the day. Despite this he was considered one of the most dangerous creatures. He is not only a ferocious fighter but also has deep connection with the Earth Mother and the Elements. He is a gentle giant and does not fear to show his weaknesses. Overall, he has very unique character and voice, and his wizened charisma would bring a very unique flavor to our roster.
Baine on the other hand is just a generic tauren chieftan; young, strong, honest, peace-loving, etc. In short, has boring character design. The only thing going for him is that he is currently an active participant in the WoW story line. I really hope they don’t bring him in because of that. Cairne is obviously the more loved character imo.