Should I use the CIM2 or CIM3 Studio Printing Engine installer?

For all VertiGIS Studio Products, which support ArcGIS Enterprise 11.x, you should use the 

VertiGIS-Studio-Printing-Engine-CIM3-x.y.z.msi

The CIM2 variant

VertiGIS-Studio-Printing-Engine-CIM2-x.y.z.msi

should only be used for VertiGIS legacy products (e.g., WebOffice) that are limited to ArcGIS Enterprise 10.9.x or earlier.

Most updates, fixes, and ongoing development for the Printing Engine are applied only to the CIM3 variant.
Additionally, VertiGIS Studio Desktop Printing supports only the CIM3 Printing Engine.

For more details, see:
What do the CIM2 and CIM3 suffixes mean for the VertiGIS Studio Printing Engine installer?
and 
What is the suggested environment to use for installations given the post installer's Esri Python library requirements?