This should say where my vote goes.
I really like HxH, but unfortunately i havent read the manga before the Dark Continent arc, I only watched the anime which was epic.
I have seen some manga panels though, and they look really bad, Togashi's art is very sketchy and nonappealing to me.
Also the latest arc is really slow paced and its getting kinda boring. Plus those hiatuses, hhhhhhhh ..... I wonder if the series is getting even finished in my lifetime. That said, its not that i hate HxH, i really like it, its just that im not that invested in it anymore. I also feel like that very investment of mine was only anime based, and not manga based. I dont know if this even makes any sense, but its how i feel about it.
On the other hand i have always loved Nanatsu no Taizai, and i still do.
What can i say, im a fan of fun fantasy adventures based in old medieval times, plus im a fan of the Arthurian legend so there is that too. I love all the Sins, and i like a good part of the side characters and villains too.
Each Sin has a backstory that is placed in certain parts of the story and every sins backstory thus far was really good or amazing. As for Elizabeth, its hard to talk about her and not to spoil stuff, so i will say this, i dont like the Elizabeth from the first and second part of the series maybe even in the third part (will have to see the end result), but her true nature and its role is cool, plus a certain flashback/backstory gives her character a new meaning.
The overall story is very interesting and pretty easy to follow, the art is great and the overall feel i get from this series is in the right place. In my opinion Nanatsu no Taizai has a fair shot at HxH.