Comment by d_christiansen

Lean occupies a different point in the design space. Its type theory is simpler and more conservative, its metaprogramming system is more reminiscent of Racket's (including hygienic procedural macros), and the focus on supporting professional mathematicians gives a different feel to the community and libraries. I think both are worth knowing, but either is great to learn. I hope the two projects learn more from each other in the future.