NT's approach to async IO is at least somewhat empirically better as it does not require an extra context switch between receiving a "ready" event and actually performing the IO operation.
Also, on an high performance poll/epoll/kevent based system you only need to poll cold fds, while you can do speculative direct read/writes to hot fds, so no need for extra syscalls in the fast case.
That doesn't mean that completion notification doesn't have its advantages, especially when coupled with NT builtin auto-sizing thread pool, but it is not strictly better.
You can do dual synchronous/asynchronous socket I/O in Windows. I use this very approach with PyParallel (and 0 byte send buffers): https://github.com/pyparallel/pyparallel/blob/branches/3.3-p...
Depending on current load, that will either immediately do an asynchronous operation, or attempt synchronous non-blocking ones up to a certain number, then fall back to asynchronous.
Described here: https://speakerdeck.com/trent/pyparallel-how-we-removed-the-...