Both would work well assuming you are using them for UI design. Note that Sketch is Mac only though. Although Figma runs in the browser, they also have Electron-based desktop apps for Mac and Windows.
Purchasing Sketch gives you one year of updates. If you decide not to renew the subscription, the app is still yours to keep and run - but you won't get any new updates. Figma follows the SaaS model of subscription, but they do have a free tier.
Sketch also has a lot more tutorials and plugins than Figma at the moment, in case that is important.
Our designers use Figma, and I've had to interact with it a fair bit as a developer. It's great. :) I can't speak to feature parity with sketch (I've used both though), but I really like the cross-platform nature of Figma, and its performance has been great for the projects I've worked with.
I have used Sketch a lot and can recommend it quite highly. I'm not a big fan of in-browser tools so it's not a knock on Figma at all, I haven't used it beyond a very short demo.
edit: I've used sketch from a design and a front end devs perspective and it was nice to use from both.
I don’t have a huge amount of experience, but Figma is a real pleasure to use. Performance
-wise it was great compared with sketch when I tried it. Ymmv
