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:
json.Decoderif your data is coming from an
io.Readerstream, or you need to decode multiple values from a stream of data.
json.Unmarshalif 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.