Michael,
Hm, yes, I have. (Of course, not with Google Chrome, since – you know – I refuse to touch anything produced by that shady company or related to it. Or, at least, I try.)
But when it comes to Vivaldi and Brave:
It depends what you want: Brave seems more like a bare-bone browser with some Goodies under the hood. You can access onion URLs with it (built-in Tor browser), it seems to have some tracking prevention built-in (untested).
Besides that, it is, almost, feature-less. (Which can be a good thing, of course.)
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Vivaldi is a completely different beast:
Forked from the original Opera, but then also switched to the Blink engine (like Opera).
I knew a bit the old Opera (with the Presto engine). It was an interesting alternative to Firefox and WebKit at that time. But, OK, once you go Chromium (Blink), you have to put something on the table, to be different. And both (Opera and Vivaldi) have seen that challenge, since they went that way.
Opera has been bought by some Chinese, don't ask me when. Opera is no longer an European thing.
Vivaldi is still European, driven by an ex-co-founder of Opera. Somewhere in Scandinavia, don't ask me where.
Vivaldi is Feature Browser. It is (probably) not optimized for speed (though I never noticed any speed drawbacks in normal usage).
It has an incredible amount of features 
Well, I start to tell you about them, but please do not think this is all:
- You can stack tabs
- You can select multiple tabs by Cmd-or Shift-Clicking
- You can hibernate tabs
- You can split the view (yeah, really). You can establish a 3-column view, or a 4-squares view, as you want, and as it fits the pages.
Many more stuff. For example the bookmark management is way superior to any browser. You can even add descriptions to your bookmarks, and shortcuts (aka "nicknames").
Brave does not offer any of this. It is not a feature browser. It is a lean browser. It is even more featureless than a stripped Chrome. (Besides the things mentioned above.)
Resume:
I don't think it is a good idea to compare Vivaldi with Brave. Brave, IMO, is a special browser that – maybe can double as an universal browser for some people. At the moment I'm not yet sure what these people are. Maybe normal ones? Not sure.
Vivaldi is a universal browser. It is even so universal that you can name it a “nerd browser”, if you wanted to do so. (But of course it is not.)
You should compare Vivaldi with Opera, not with Brave:
Vivaldi is a bit like the "old" Opera before they gave up the Presto engine. Of course, nowadays both are based on Blink by Google (a WebKit fork).
I know it is cheap, but I say it anyway:
Try out both, Opera and Vivaldi, and see what fits you.
When doing this, keep in mind that Opera is trying to trick you:
They say the browser comes with an integrated RSS reader. True, but the reader is crap. You get better ones from the Chrome store (with a similar memory footprint), or you just rely on an external news aggregator. So: Nonsense.
They say the browser comes with a VPN. Nonsense also, since a VPN can hardly be limited to a specific app (browser). It's just a proxy.
It comes with an integrated ad blocker. Fine, they say it is faster because it is better integrated. (Will be executed before the traffic arrives, etc.) Is it true? I doubt it. Vivaldi with uBlockOrigin is negligibly slower with the Jetstream2 test.
The "Flow" feature? Well, I don't know. Try it. I don't like it. (It has some sync uses.)
I think you got the idea. If not then just go ahead and try the browsers.
Opera seems to be in a bad state currently (technically)
Try out both browsers or even the Brave browser.
Just do one thing before: Export your bookmarks as an HTML file and guard it like your loveletters. This way, as long as you guard that file, nothing bad can happen. Because, as we all know, your browser is just as good as your bookmarks.
Edit 2019-06-06 T 2055:
- Removed some late-night political incorrectness
- Corrected some minor stuff
- Added links
