Vivaldi lets you tweak every button and theme, so if you're someone who absolutely needs to customize every corner of your browser, it actually gives you that control. The downside is it looks and feels old, and what used to make it special is now just table stakes in every other browser. Still, if obsessive customization is your thing, Vivaldi delivers.
Sidekick, on the other hand, doesn't bring anything unique anymore. Its so-called productivity features are now standard everywhere else, and the design experience lags behind even other modern browsers. Plus, it's been shut down, so you're basically picking a dead end.
If you want control over every detail, Vivaldi is the only real option here. There's just no reason to pick Sidekick now.