Bublé creator here. The obvious big difference is that this supports TypeScript and Flow, which Bublé doesn't. Bublé is designed to compile a subset of ES6+ into ES5 (the subset of ES6+ that transpiles well without causing bloat, which excludes some features like generators), whereas this seems to be focused on more modern targets.

Technically there are a few interesting differences under the hood, such as Sucrase not wasting time generating a full-blown AST (which Bublé needs to do in order to handle things like block scoping).

Anyway, I won't waffle on any further as I'm sure Alan can explain the differences better — I just wanted to chime in to say that I'm excited about Sucrase. I've written a [Rollup plugin][1] for it, and I'm planning to use it with my TypeScript projects.

[1]: https://github.com/rollup/rollup-plugin-sucrase

Adding to what Rich said, Sucrase takes a fair amount of inspiration from Bublé, and has a similar high-level approach of "pick a well-scoped subset of what Babel solves, and focus on doing that really well". In Bublé's case, that's compiling many (but not all) ES6 features down to ES5. In Sucrase's case, that's compiling language extensions (plus some upcoming JS features like class fields) to modern JS. They solve different problems, so you would never pick between the two.

Architecturally, they both skip a bunch of work that Babel does (AST transformations and AST formatting), but Sucrase is a bit more ambitious. Bublé does a normal parse step and then does replacements using magic-string. The original hope was that Sucrase would be able to skip parsing and just tokenize the JS and do all of the necessary transforms from the tokens. It turns out that that's basically impossible, but Sucrase still tries to find a middle ground where it does something reminiscent of parsing but doesn't need to produce an AST at the end. It's still able to resolve variable scopes (which is needed for the TypeScript and imports transforms) by providing an array of scopes in addition to an array of tokens, and it provides enough useful context in the tokens to put the pieces together if necessary.