If it's between those two, I'd go with stamina until you're either overcharged or find yourself getting very low on health when fighting stuff. Put some in health at that point.
It depends on how many nightblade skills you use(magic) and how many archer skills you use(stam). I'm going Nightblade assassin and at the moment it's more magic than stam.
Unless you are going to be questing with others, I would suggest an even split in the beginning levels. I tried dumping it all into Stamina on my siphon archer early on and it made me pretty darn squishy if you are relying on archer skills to do most of your damage - at low levels, they definitely kill slower than many other things. You're going to get melee'd.
Just an update as I've leveled my Siphon/Archer more. It is just the very early levels that health is a problem. I've started veering towards a 1:1:3 attribute spread now.
In PVE at least I find most damage to be avoidable or reduced significantly with good timing on blocks and cc. I have yet to spend any points on health leveling. I dump everything into stamina or magicka (or balance them evenly if I'm doing a class/weapon split).
This is particularly true with nightblade archer if you're setting up your kills with rear sneak attacks for the opening stun, and pick up the spell interrupting poison arrow. You can really put a hurting on things without being touched.
I say balance them out until hit about 10 (it allows you flexibility to experiment with class/bow moves). If you find you're dying before you're even out of stam or magic, then beef up your health.
You *can* avoid taking in health, but if you get dotted, you're going to have some close calls (at least until you can morph your invisibility skill to cleanse you)
Holgrum put some pants on. Holgrum heal me I'm dying. Nag... Nag... Nag...