I've wondered about that too. Do the Japanese say "CD-ROM ROM" too? What did TTi use? Squared is probably more correct but yeah it feels awkward to say. Coming from Sega CD land, it's a bit confusing seeing the acronym "SCD" being used.
So I think I'll actually just say "Super CD" as suggested, but make a comment about the alternate pronunciations. Also, thanks for showing the ad, it really is quite... special.