Hi Blake,
If you work with Geoprocessing services, an input featureclass will be send as FeatureSet to the service (in json). The IN_MEMORY workspace is, as the name suggests, a workspace that is available in memory to write (intermediate) results to. Since a FeatureSet is a lightweight abstract of a featureclass, it does not support all the methods a (in_memory) featureclass does. Both are used in different ways for different purposes.
Maybe you can explain in what type of process you would like to use them to be able to give some suggestions on what type to use...
Kind regards, Xander