To summarize the differences between CorelDRAW Graphics Suite and Corel DESIGNER Technical Suite, the first one is designed with the creation of crative graphics in mind (logos, flyers, brochures, signs, ...) while the second product is focusing on technical graphics and illustrations (graphics for parts manuals, working with CAD files, ...).
Corel DESIGNER Technical Suite extends CorelDRAW Graphics Suite with enhanced filter support for CAD file formats, DSF files, ... and adds features such as projected drawing (to create 2.5D Isometric drawings), call-outs, dimensioning, precision drawing tools.
As a side note, we currently have the pre-release technology preview of the next version of Corel DESIGNER Technical Suite (X4) available as an exclusive for the CorelDRAW.com members. You can find all the details under http://DESIGNER.com/preview. A few things that we are working on for DESIGNER X4 are 3D CAD formats support, improved DSF compatibility, enhanced diagramming tools, ...
Gérard