Before I state my opinion on this, let me just say that I always recommend Nurgle for everything.
That said, I think Undivided would be best for neaky Marines. I don't think any of the four colors lend themselves to stealth at all, really. You will smell Nurgle and see things rotting around you. Thousand sons are just ghosts in their armour that can' really learn skills like stealth (although you might say that a sorcerer aids the whole stealth thing with spells, but I think that's kinda like cheating). Slaanesh likes loud noises that take them to the next level of audio-potence, and of course there's the disturbing moans and screams from... being Slaanesh... And Khorne, well, charging headfirst and screaming "BLOOD FOR THE BLOOD GOD!" does not help you avoid detection.
Undivided, however, is where Legions like the Alpha Legion and Night Lords are, and they're good at stealth. So that's where I'd go with the Storm Marines. Just remember that it's given that I vote for Nurgle first.
__________________ Lord of Fluff and Blood Angels Herald of Nurgle "I wield my power with the Emperor's Authority. Those who would say that I am 'radical' merely have minds too small and impotent to realize all the weapons at their disposal. Do not question my methods on account of these so-called 'Puritans'." -Inquisitor Mathias Rosenadel |