Use "Full".
If you are using CoreOnly, then you are using the Google code download. In the root of the zip is a file called "what files do I need.txt" (or something similar), which explains what CoreOnly is, and how to use it. The short version is that it doesn't include the meta-programming layer, and is intended to be used with the precompile tool. The methods directly on Serializer are tied to the default, meta-programming-based model - hence are not available in CoreOnly.
The main use of CoreOnly is for environments like mobile devices where meta-programming is not possible. Actually, it is usually easier to use the NuGet download, which only includes the Full version.
I am, however, tempted to add these methods, but with an [Obsolete("some text", true)]
that explains all this - to avoid future confusion.