At the moment, PNaCl supports pthreads-style threads and asm.js does not. Also, Native Client, but apparently not PNaCl, supports SIMD, while asm.js does not.

At the moment, PNaCl supports pthreads-style threads and asm.js does not. Also, Native Client, but apparently not PNaCl, supports SIMD, while asm.js does not. Of course, both PNaCl and asm.js are evolving, so asm.js may be able to eventually support those features somehow, and PNaCl may get additional features. Some information about features PNaCl supports are here: https://developers.google.com/native-client/dev/faq https://developers.google.com/native-client/dev/faq