System.Net.Http.HttpClient: It is available from Windows, Windows phone 8 onwards,we can USE this If you are going to develop app with cross platform such as iOS, ANDROID. This API sppports Xamarin IOS and Android development. 

Windows.Web.Http.HttpClient: It was FIRST introduced in Windows 8.1 and was available on Windows Phone 8.1 as well. So PRIMARY focus it can support all windows STORE languages(C#, VB, C++, JavaScript) into a single one that supports all the features from each of those APIs. We can use this If you are going to develop native UI or pass specific SSL certificates for Authentication.

System.Net.Http.HttpClient: It is available from Windows, Windows phone 8 onwards,we can use this If you are going to develop app with cross platform such as iOS, Android. This API sppports Xamarin IOS and Android development. 

Windows.Web.Http.HttpClient: It was first introduced in Windows 8.1 and was available on Windows Phone 8.1 as well. So primary focus it can support all windows store languages(C#, VB, C++, JavaScript) into a single one that supports all the features from each of those APIs. We can use this If you are going to develop native UI or pass specific SSL certificates for Authentication.