Hi there!

So far I have been using a "speedy" in my battles - a Von Carstein Vampire Thrall with Wolf Form, Great Weapon and Flayed Hauberk. He did well in many a battle, he sucked badly in equally many. It made me consider another option - a Wraith with Obsidian Amulet. Though a Vampire has 1+ AS and T4, it seems this guy will somehow be even more resilient. And he spreads terror. And ignores all obstacles. Downsides are he has less attacks, less strength, less movement, less WS.

Both are supposed to hunt on their own, not to remain a part of the unit. Which one should I choose?

If you are intent on keeping him on his own I would say say the thrall since his greater movement can get him out of trouble if needed. however if the wraith gets bogged down in combat (fast moving troops) he will die, even with the obsidian amulet. no, wraiths are better of in a big unit of zombies/skellies. this also apply to thralls due to the undead rule.

Do the Mounted Wraith with Talon if you want a lone hunter.

The 4+ Ward is, really, still 4+. An average of 7 hits from a single Master of Stone is still going to kill him badly. Not very resilient, really. On an ethereal Master Necromancer, if it's there at all it's there as last ditch protection, in *case* the Zombie Escort Agency fails (which is highly unlikely). Not an impressive item generally.

The 16" charge, Terror causing Wraith that kills 2 war machine crew before combat even starts is far more useful.

The Wolf Form Thrall is generally good, and is definitely not immune to bad luck. Occasionally it won't take down what it's supposed to - but sometimes I find it's a matter of expecting too much.

Try also the Wolf Form BSB, or the Blood Dragon BSB. The first can also be a von Carstein Sword of Might BSB. Very, very good lone hunter type.