Fencedude said: "Seiren", which is also closer to how its pronounced in the show.
It's also how it's pronounced in the original Greek- it was warped somewhere on the way from there to English. As a loanword, it seems to be spelled variously as セイレン and セイレーン - but as サイレン only when referring to klaxons.
For that matter, I think YamiBou's seiren should be Siren too (the inspiration is quite obvious in her case if you're familiar with the story,) but I have fansubber precedent working against me there.
I think we should probably stick to using kurokawa_ellen for all of her humanoid depictions, and start using siren_(suite_precure) as a transformation style tag for her cat form.
We could alias siren_(suite_precure) to kurokawa_ellen like we would for most late name reveals, but due to the presence of Hummy in a lot of her pictures it would make it pretty much impossible to properly search for the handful pictures of her cat form that exist.