Mitch, much of this is terminology verbiage now. To start with, Essential ESU line is the fairly new product that will be shortly replacing the Concept DCC paint product line. I/we have not changed over to the ESU line of substrates because we are still using the MP line however we are starting to shoot some of the ESU finishcoats with favorable results.
With regard to the word "primer", the basic jest is that the Epoxy is a thin "stick-to-everything" substrate designed to go over bare sheetmetal that will promote adhesion for whatever substrate is to be applied next over the top. The downside is that an epoxy generally does not sand well, and it is very thin. If a bodyman is going to install a new fender that may (-or may not) be powersanded, then this is a great product option as light scuff sanding is all that is necessary to get mechanical adhesion. A polyurethane generally has more solids meaning it is a better product to be used as a surfacer, --or for leveling (blocksanding). Urethane primers generally sand much better/easier too.
There used to be some hype that you should use an epoxy first under a urethane ...especially if there was going to be some length of time between the spraying and the bodywork phase however that is not necessarily the case any longer. Now PPG is actually recommending that all bodywork fillers be applied directly over the epoxy primer. Since what we generally do is outside of the mainstream of collision work, their product recommendations are often written where it is confusing to the Restorer. Having a PPG rep or attending their schools is about the only way to learn "the other information" that they don't print. In your case, you will be OK with whichever product you choose however in an effort to save money from needing to stock both products, I would go with the higher-build urethane primer.
