Written: NES, Nintendo, SNES, Super NES, Super Nintendo
Spoken: N-E-S, Nintendo, Super N-E-S, Super Nintendo
I've never said "Ess-en-ee-ess" when referring to the SNES. It takes just as much effort to just enunciate "Super" with the addition of not sounding like a snake.
Saying "Ness" or "Sness" just sounds stupid. I would probably assume (rightly or wrongly) that anyone that pronounces it like that wasn't around back in the day. As far as I remember, nobody called it that. It's as ear-cringing as hearing someone call cartridges "tapes". If someone were to say "Ness" or "Sness" to me in real life I'm sure I wouldn't know WTF they were talking about at first.
Maybe it's a UK/Irish/European thing, but we all called it nes and snes.