Are these both one-in-the-same or entirely different from each other?
They are the same. It’s just that the script transcoder got converted to a plug-in.
So the same ssc_codectypes and ssc_script apply. If you use the default transcoding script (mt-daapd-ssc.sh), then it uses oggdec and flac command line decoders.
Since flac 1.1.1, there has been a “–apply-replaygain-which-is-not-lossless” option which will decode with replaygain.
Not sure about ogg.