Ill post it if you could add in a snippet of background on both Emhyr and Radovid
Certainly.
For more, in-depth
spoilers of those two men, here are their wiki pages:
http://witcher.wikia.com/wiki/Emhyr_var_Emreis
http://witcher.wikia.com/wiki/Radovid_V
Geralt can help (directly or indirectly) each one by thwarting the other's plans and at some point, play a significant role in the assassination the other.
Now for the snippet.
Emhyr and Radovid are the rulers of the kingdoms of Niflgaard and Redenia, two of the 4 most prominent nations in the Witcher universe and the biggest nations in the current "Nord War" that takes place during the events set up in the
Witcher 3's time.
Emhyr is the aggressor in this war, the third Aggression war of the North to unite all the South(ern) territories under the Niflgaard banner and sovereignty, something he had set up in doing in all 3 previous wars.
He is also Ciri's (Geralt's trainee and protegee) father, who sets up the events of TW3 by summoning Geralt and task him with finding Ciri that is pursued by the Wild Hunt, the main antagonists of the story.
He is megalomaniac, cold, stern, unforgiving, even to his daughter, and like it had been said, he is the aggressor of this war. He wishes to expand his empire and unite all of the North and South under the Niflgaard standard and he has waged three wars so far (2 of them failed) to succeed in this endeavor.
However, if Geralt chose to help him, he becomes a despot more than a tyrant, even granting Temeria (Geralt's and co. nation and the biggest economy in the world at that time) a limited autonomy in their rulings. Plus, the non-human portion of the populous gets to enjoy the same benefits as humans. But no matter how golden a cage is, a prison is a prison.
Radovid on the other hand, you realize what kind of person he is when you enter Novigrad, a free city within Redanian's boarders, where you see various people burned at stakes for either being non-human or being accused of witchcraft.
Radovid has a deep-seated hatred for wizards ever since his father was assassinated and a witch was the mastermind behind all. And the current war that he is (currently) losing against the Nilfgaard empire has left him suspecting everyone and everything of treason and/or witchcraft inside his boarders. He is a lunatic, unhinged, suspicious of all, quick to anger, but, one of the most brilliant strategists and minds of his kingdom, even on his time.
If you decide to side with him, the war ends, no more war, but that does not mean no more suffering. He bestows an ethnic cleansing on all the nations under his influence, everyone who is either a wizard (even herbalists are accused of this title) or a non-human gets burned at the stake! However, Radovid's rule, regardless of how pogromish it may be, will be short-lived.
So that is your choice on who to help:
Help the guy that hates and wants to subjugate anyone who is not Nilfgaardian.
or
Help the guy that hates and wants to subjugate anyone who is not Radovid.
Hope I helped.