Considering the dubs of many games are known to mispronounce things, I wouldn't rely on the way Psycho Mantis said it. Also I seem to remember him pronouncing it "Soo-ee-koh-den"
eek Edit: Yup, mantis does say it like "Soo-ee-koh-den"
http://youtube.com/watch?v=Bc-K6wMBFkY3 minutes and 5 seconds into it for those lazy bones out there.
xp His accent is what makes it sound like 'soy.'
Although as I mentioned before as long as people know what you're talking about it doesn't matter how you pronounce it much.