Only a very strange person thinks that these are all totally different trucks!
IMHO, it's the same truck in all the cases, just with some options that I can choose to buy, or not, depending upon how I want to spend my time "playing".
The real "difference" comes when I'm deciding between a Ford, a Chevy, or a Toyota.
Surely you are looking at different lines of vehicles from those manufacturers, right? A Camry vs. a Carolla, that kind of thing.
Assuming that you are, here's a question: how different does a car have to be in order for you the consumer to accept it as belonging in an entirely different line, and not merely being a variant? Will changing the body alone do it? Will changing the stuff under the hood alone do it? It's not an easy question to answer, and I think we could get a range of subjective takes.
My point is this: if the manufacturer added something to one model that more than doubled its price and gave it the ability to do something really fantastic that no other vehicle could do, would they be justified in giving it a different name and selling it as its own line even if all the other stuff was the same as another line?
I think they would.
This is why dismissing the CD drive as "just storage" like it's some minor option is doing it a disservice IMHO. From a user-centric viewpoint, especially in the context of the time, it practically makes the PC Engine into a car that can drive underwater. A little sound upgrade, like the Master System's FM module, would be a minor option, but they weren't selling this as a little sound upgrade. One Japanese ad I saw said "This system can hold games 1000x larger than Dragon Quest 3!"
Now, did Hudson and co. brand the CD system as a new line? Well, no. Branding, frankly, is the main reason why I think old-timers say it's all PCE.
Alternatively, was Sega's "Welcome To The Next Level" nothing but hype, and the Mega CD nothing but a Siamese twin-head that suddenly sprang out of the Mega Drive's collar one day? Perhaps it was.
And perhaps it mostly doesn't matter!
Mostly, I don't think about this at all. The only reason why I'm giving my two cents now is because I believe that if you're going to lay out the history for future generations and newcomers like StarDust4Ever, it's
nice to draw a big fat boundary between the Hucard system and the CD system. I think that anybody who expects this to be like a console and not like a bloody PC, whether they came from a Sega-oriented background or not, would find it odd and confusing to think of all this different crap as
one thing.