Oof, that's a really tough match-up. Honestly I'd vote for both if I could.

They're just about even on plot, but I'm gonna have to give it to The Witcher, for the simple reason that it allows itself to focus on smaller character moments a tiny bit more often than FFVII, and those character moments tend to land better for me in The Witcher.

It's trickier in the villains category. Sephiroth is a great villain, and the main villain of The Witcher III's base game is terrible. I don't even remember his name or what he looked like. He's really just a walking plot device. However, the villains in The Witcher III's DLC blow Sephiroth out of the water. In Hearts of Stone, Gaunter O'Dimm is equal parts creepy and powerful, and he might be one of my favorite villains ever. In Blood and Wine, Dettlaff and Syanna have a fascinating dynamic. I feel compelled to give the DLC less weight since it's optional, but I think it still ends up as a tie.

Side content, though, is not really a contest. The side content in FFVII is great (Vincent and Yuffie's storylines are the highlight for me, and there are some great optional bosses), but almost every single piece of side content in The Witcher III tells a compelling story. Its side quests are riveting, to the point where they're quite frequently better than the main quests.

As much as I hate to vote against FFVII, it's gonna have to be The Witcher III.