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Vivaldi lets you move every button exactly where you want, while Zen Browser tries to avoid Google and Chromium but breaks extension support and can be buggy on many sites.
Only use Vivaldi if you care about customizing every detail; skip Zen Browser unless you absolutely need to avoid Chromium and accept limitations.
If you care about browser extension support, Vivaldi is the safer pick because it's Chromium-based, so nearly all Chrome extensions work out of the box. Zen Browser, built on Firefox, will leave you missing most of your favorite extensions, only 2 out of 29 ported over in testing, so you'll probably hit dead ends if you rely on a lot of add-ons.
Vivaldi also avoids the site compatibility headaches that come with non-Chromium browsers. Modern sites are built for Chrome first, and Zen Browser users will run into more bugs and weaker developer tools just because it's not Chromium underneath.
Zen Browser only makes sense if you're a die-hard Firefox fan craving a sidebar experience like Arc, and you're willing to live without most extensions and put up with more site quirks. Otherwise, Vivaldi just works better for most people, especially if you're coming from Chrome or care about things not breaking.
