Currently JavaScript is the default for starters in Ionic 2 but this is going to change.
While I personally prefer TypeScript, there’s definitely a lot to be desired in terms of library support (e.g. there are still plenty of missing or outdated type definitions), especially for the less popular libraries (but not only). It’s also a matter of personal preference, so I would suggest you to learn more about TypeScript first and check some code written in it. There is a good article about TypeScript.
In the end the best way to decide is to probably make a small app in TypeScript (or find a tutorial for such an app in JavaScript and recreate it in TypeScript) so that you can feel the difference and decide on your own.