When referring to the Sega CD, I almost always type out "Sega CD" since not typing 3 measly letters doesn't save much time or effort. When referring to the PCE or TurboGrafx, I usually type TG-CD, TG-16 CD-ROM, PCE-CD and the like. I also call the SMS an SMS, unless I spell out "Master System". I'm usually not one to overly abbreviate since I have the power, the ability, the effort and the manliness to type things out... on most occasions.
If someone typed "SGCD" I would think "SuperGrafx CD". MCD works great in the areas where it is called the Mega CD, but in the US it is called the Sega CD, not the Genesis CD. That's why many type out "SCD". No matter what, nobody owns the acronym, so it doesn't legitimately belong to one system or the other. Getting upset about it is pretty silly. I rarely say "Super CD". I just refer to Turbo/PCE CD games as CD games since I can play them all, and it is worthless to distinguish unless it is an Arcade Card game and I have to switch cards.