Is it wiser to learn Sass instead of Less amidst Bootstrap 4 release?

Bootstrap 4 was built entirely from Sass, which isn't surprising considering how much traction it's had over the last while. There's almost double the userbase that supports Sass, there's more syntax clarity, the Compass library is pretty solid, just to start. Given that Mark Otto himself has said a lot of the same things and more, I'm banking on Sass being around for a while in the mainstream. Less will be around, sure. Some people don't like change, some people still like what it's good at, but like with most things like this, it's largely driven by what's in play now. If you're aiming to stay relevant and have skills to make you valuable, I'd aim for Sass, or heck, even add on some PostCSS (nodding at the tweet I linked). There's still some value left in Less, and you never know when you'll have to plow your way through Less's territory. Less will still be around, and who knows, someone might pick it up and turn it into something better and drive more people to use it for more things, so it's not like you're wasting your time. I would keep it under your belt, maybe treat it as a weekend project, use it while you've got it and keep it fresh, but I'd definitely consider following the trends with your workflow as well.