What is the difference between an ESRI Printing Tools print and a VSP Engine Tools print?

Print requests for VSP Engine Tools prints are 100% compatible with requests for ESRI Printing Tools, since both printing solutions are using the same specification: https://enterprise.arcgis.com/en/server/latest/publish-services/windows/exportwebmap-specification.htm 

This means that the VSP Engine Tools have the same capabilities as the ESRI Printing Tools, but these capabilities are extended in various ways to better fit the requirements necessary for distinct use cases. 

One major benefit is that the VSP Engine Tools are capable of processing MAPX based print request from VSD as well as webmap based print request from VSW. The Engine features however are used differently by the different VertiGIS products. VSD is using the Engine for generating simple prints as well as for generating potentially long-running Map Book PDF documents with up to 100 and more PDF pages, containing multiple print types and layout template formats within a single output. VSW currently only supports simple prints (as of VSW version 5.30, VSP 5.24).