I think Shurima would win both.
At first round, Shurima is a bigger region, so It is likely they can bring a lot more human soldiers than Freljord. And not even counting sandsoldiers of Azir which he can remade again and again. But it would also depend on if fight is happening on a neutral domain or not.
Nasus and Azir overhelm Lissandra, and other characters like Ashe, Sejuani, Trundle.
Second round, it is kinda tricky. We don't how mighty a spirit god is, or how well an Ascended match against them. But we know high tier Ascended are capable of killing spirit gods, though I believe Ornn, Anivia and Volibear are high tier among spirit gods too.
I believe Ornn, Volibear and Anivia as manifestations of nature are slightly stronger than Nasus, Renekton and Azir, beings transcended by power of cosmic energy. But Xerath breaks the trick. 4 Ascended versus 3 spirit gods. And Xerath is arguably the most powerful among all of them, even spirit gods.
I believe Xerath can handle 1 or 2 spirit god(s) (Aatrox was capable of killing spirit gods) and other 3 would be more than enough to defeat 1 or 2 spirit god(s).
Still, second round is not certainly in favor of Shurima. It is possible for Freljord to be victorious, but I still give my vote on Shurima. Renekton can match Volibear, Azir can match Anivia, Nasus can match Ornn in best scenarios.