It really depends on what your input is. If you look at the implementation of the Decode method of json.Decoder, it buffers the entire JSON value in memory before unmarshalling it into a Go value. So in most cases it won't be any more memory efficient (although this could easily change in a future version of the language).
So a better rule of thumb is this:
- Use
json.Decoder if your data is coming from an io.Reader stream, or you need to decode multiple values from a stream of data.
- Use
json.Unmarshal if you already have the JSON data in memory.
For the case of reading from an HTTP request, I'd pick json.Decoder since you're obviously reading from a stream.