Depends on what you need.
If you think you need more self-sustain (healing) in longer fights with big baddies, go for Blood Craze.
If you are playing alone or in a group where no-one else can reduce the target's defenses, go for Reaper's Mark. It's also really good against big groups of smaller monsters as you get a huge heal every time you kill your marked target.

*Edit: Yes, healing from different abilities do indeed stack. You could also throw down Refreshing Path (Path of Darkness Morph) for a third source of healing and area damage if you so desire.

I leveled up as a Leech Tank with my Nightblade, the build was as follows:
(Theorycrafted during early Beta and tweaked along the way)

Full Heavy Armor (Perhaps throw in one piece of Medium and Light to level up those skills while questing)
One-Handed & Shield Bar:
Entropy, Strife, Siphoning Strikes (I highly recommend Siphoning Attacks), Puncture and Refreshing Path. Though you might also want to try out Sap Essence (Drain Power morph, it's very powerful against groups of monsters and gives you a nice damage boost even against one.) Ultimate is very much up to you, but I highly recommend Soul Tether (Soul Shred Morph), it's insanely powerful in 1.6.

For my second bar, I had a bow setup that I would use when on the move or to initiate sneak attacks, though you can use anything you'd like and it'll probably work, a dual wield setup for example:
Snipe, Poison Arrow and Concealed Blade (Veiled Strike Morph), Cripple and Shadow Cloak.

The idea behind the first bar is to be able to survive with your huge defenses and self-healing (I was able to solo some world bosses and most of the group delves even in Veteran areas). Remember to block incoming big hits!

Second bar was my sneaky setup. As you might have guessed, the build is somewhat of a mixed one. I had a bit more magicka than stamina, but overall it was pretty even between them. I also invested some in my HP. You can deal some serious sneak damage by getting close using Shadow Cloak and then striking with a fully charged Heavy Bow Attack together with Concealed Blade (draw your bow and hit Concealed Blade to make them both hit for sneak damage). You can also skip groups of monsters - even whole delves - with the usage of Shadow Cloak along with Vampirism Sneaking speed buff and Concealed Blade as you sneak almost as fast as you sprint.

Main point being: Experiment! Come up with a cool idea for a build and try to make it work.

I apologize for the lengthy post, but I hope you got something out of it!

You are welcome.

If you're on the EU Mega Server and have questions, don't hesitate to send a message to @Aerieth.