edit: I've used sketch from a design and a front end devs perspective and it was nice to use from both.
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.
Used to work in an ad office making ads with Freehand and Illustrator and Photoshop. Figma is a wet dream compared to tools of that day. Could not have even dreamed of something like it back then.
Illustrator on the other hand is still vastly better at what it's named after - illustration. Figma has great basic vector functionality that will cover 99% of your design needs, but little around illustration needs.
I'm really excited for Figma and what comes out of it, but it still has a long way to go to catch up. I think it has a better foundation though.