As someone who sometimes has to use the POSIX "API", let me tell you this: it's a piece of a fucking joke. As much as it pains me to say it, the functionality provided by the core winapi, even with its idiosyncracies, is way ahead of anything POSIX lets you do. Sure, you do want to use a decent C++ wrapper around either winapi or POSIX, but still: POISX doesn't do the very basics of what you need to develop actually useful applications: you need to provide lots of your own, or library, code to get where you want to be, foundation-wise. People hail POSIX as something wonderful, but all I see is a bunch of very much anti-UNIX philosophy codified for no good reason. Even Linux APIs are much nicer than POSIX - the modern "everything is a file descriptor" is much closer to what Windows provides with its universal object handle metaphor. All the while POSIX has special approaches to everything: when you wait on one thing, you can't wait on another thing, everything needs special treatment - it's disgusting, frankly said.
POSIX api was designed by people who never seemed to realize what the needs of modern, responsive application and server design are.