> What's the difference?

One lets you take existing JS-code and make use of new language features there. The other "just" lets you call existing JS-code.

I'd guess one is clearly better than the other when it comes to dealing with existing JS-code* . (While the other is clearly preferable to the one when it comes to making the language you want to make.)

* It still obviously depends a lot on the language. Like, how well TS-code works together with plain JS-code (how much do you get out of adding some type annotations here and there while leaving most of the JS-code untouched, etc.).