been avoiding both for a while until I see a need to do beyond basic analysis on the web. The ArcGIS API for Python | ArcGIS for Developers seems to have stepped up options for this, on the analysis side, given scipy et al. are actively mentioned. I troll around Esri GitHub | Open Source and Example Projects from the Esri Developer Platform to see what is going on occasionally to see which way the wind is blowing on the developer/example side.
I can mention one important difference between two APIs: ArcREST can be used for administrating ARCGIS Server, and for executing geo-processing services, querying map services that resides on your own ARCGIS Server. ArcGIS API for Python until now is a platform for manipulating Portal, it can not be used for direct access to ARCGIS Server. So it can't replace ArcREST.