One of those is JavaScript, the other is a dialect built on top of it. I'm sure there are many advantages of TypeScript when it comes to testing debugging your applications because of the additional typing options, but I think it would be hard to argue to build it in not-JavaScript. I've never used TypeScript to take my opinion with a grain of salt, but I've heard from a few TS users that TypeScript is the sort of thing every programmer should do at least once to understand the principles, but not something that you need to use for every single project once you have learned the "why" it exists and "what" problems it helps you avoid so you can avoid them yourself. If you haven't used TypeScript yet, there are a bunch of developers out there who I'm sure would tell you to do this project in TS just to learn about it - maybe that fits in your project goals and timeline, maybe not.

considering that most of your code will be running in NodeJS i would recommend using ES6.

while its absolutely possible to use typescript with node, i think the benefits it offers would be outweighed by the downsides of having to compile it, and having to use source maps when debugging.

also node 6.5 onward supports 97% of ES6 features. kangax.github.io/compat-table/es6