In a nutshell, WASM is slightly more efficient than Empscripten + Asm.js because it has a binary format and circumvents the need to parse a blob of JS. That's about it, really.
Whereas Asm.js had (has?) perfect backwards-compatibility with unsupported browsers and JS interpreters, WASM requires users to remain on the bleeding edge of new browser features as it continues to evolve, and introduces a whole host of fantastic new bottlenecks as the designers puzzle over how to interface WASM modules with the rest of the facilities JS can already access.
The whole thing is a hilarious boondoggle- an insane amount of effort and complexity for mild bandwidth and page-load time savings- made all the more hilarious for the fact that a remarkable number of people seem unaware that Asm.js ever existed in the first place.
Whereas Asm.js had (has?) perfect backwards-compatibility with unsupported browsers and JS interpreters, WASM requires users to remain on the bleeding edge of new browser features as it continues to evolve, and introduces a whole host of fantastic new bottlenecks as the designers puzzle over how to interface WASM modules with the rest of the facilities JS can already access.
The whole thing is a hilarious boondoggle- an insane amount of effort and complexity for mild bandwidth and page-load time savings- made all the more hilarious for the fact that a remarkable number of people seem unaware that Asm.js ever existed in the first place.