SGK:
Pros:
- Relatively Cheap Blender
- Useful if you're planning a scream-based list
- Has spare item/power points for some surprises (if you're playing with closed lists) - e.g. flying
- Can be a nice caddy for an arcane item
- Um... nice model
Cons:
- Much more fragile than a Vampire Lord, and especially vulnerable to Killing Blow and Death Magic
- Can't take a mount
- Can't take mundane or magic armour
- Basically limits your casting to Lv2s in the Hero Section
Vampire Lord:
Pros:
- Can be kitted out as both a blender
and a lv4 wizard
- Can be made very durable with a 4+ ward and mundane armour (or Nightshroud and a 5+ ward).
- Can take a mount (not sure how practical/useful that it though)
- Very strong all-rounder, really.
Cons:
- Unlikely to have many points left over to carry an arcane item or utility power.
- You may have to compromise his blending ability very slightly (compared to the ghoul king), or have him as a Lv3 wizard in order to get the most out of his blending ability.
- It can be dangerous to have your main caster as your general - one bad miscast roll can doom your entire army.
Master Necromancer:
Pros:
- Cheap
- Plenty of room for an arcane item (e.g. Black Periapt, Earthing Rod), as well as a ward save.
- Gives good magic defence
- Can be safely bunkered behind the lines
Cons:
- About as useful as a Jam Sandwich in combat.
- Basically limits you to hero level blenders, which are decent but also very fragile.
- Having your main caster as your general makes miscasts very risky (although, not to the same degree as a vampire lord - because you can buy an Earthing Rod and lose far less if he does die).
- Can't take any vampire powers
There is one other thing you could consider - which is a Ghoul King AND a Master Necromancer at 2000pts. [The hyperlink button appears to have committed suicide, but if you're interested there's a thread with that title in the Teachings of Aborash]
Not sure how competitive it is, buy I thought it was worth mentioning in case the idea appeals to you.