Which System.Net.Http do I use?

In VS Mac, my Xamarin.iOS project shows 2 references to System.Net.Http. One is coming from the References node with a location of:

/Library/Frameworks/Xamarin.iOS.framework/Versions/Current/lib/mono/Xamarin.iOS/System.Net.Http.dll

and a version shown as 4.0.0.0.

The other is coming from the Packages node via Nuget with a version of 4.3.4.

Do I need both? What is the difference between them? Which one takes precedence?