[webjprgm]

Does Typed Racket have DrRacket support for instant feedback on errors? (It didn't 4+ years ago when I used Racket.)

Typed Racket also includes more types, like ": Integer [more precisely: Negative-Fixnum]" (from docs), as well as polymorphic data structures and higher-order functions. I don't know if this is a "fundamental difference" but it might mean Facebook's type checker can optimize in certain ways that Racket's cannot.

[samth]

Yes, Typed Racket does have that -- DrRacket continuously expands in the background, so Typed Racket gets it for free by integrating into the macro system.

The Typed Racket type checker is much slower than Hack, though.

Hack does seem to have polymorphic data structures and higher-order functions.