I have had both - Nero Noctis is literally a non-metallic, "pure" black. It is the most gorgeous black I have ever encountered - deep, deep, deep, and properly maintained, it absolutely glows. But, therein lies the rub. It is also the most easily swirled and marred paint I have ever seen. It seems like every little contact with its surface results in swirls. I feel that the ideal situation with Nero Noctis is to detail it (or have it detailed) perfectly, then cover the entire car with paint protection film (XPel Ultimate Plus is stellar), otherwise, if you are at all OCD, it will drive you mad.
Nero Helene is as you say, metallic. It is an interesting metallic, in that the flakes are multicoloured. To be honest, it doesn't have that rich, deep glow that Nero Noctis has because of the flake, but also because of the flake, it has its own positive attributes - especially up close, the flakes do look good.

Here is a pic up close of Nero Helene:



(Apologies - it was difficult to get the camera to focus on the paint itself)

Here is a link to a detail that I did on a good friend's Superleggera. His wash technique is great, yet his paint had a multitude of swirls in it. ANY cars painted in Nero Noctis that I have seen are swirled, and quite badly, at that. The pics kind of show what the paint can end up looking like even with careful washing, and what it can look like after a detail.

A special Gallardo Superleggera...

Hope this helps, in some way.

If you can maintain it properly, Nero Noctis is stunning. Nero Helene hides defects better. After owning both colours, I will be going with Noctis for my next car!