-sighs- Unfortunately, Necromancy's a long-denied branch of magic in the rules. Dread Necromancer or Cleric's about the only way it works (yes, I laugh at the specialist wizard option and mock its pants and eyebrows). So, generally, no matter what you do you're gonna feel the pain of having the rules against you. Either way, both classes only really pay off if you stick to one or the other, particularly Dread Necro. True Necromancer's not really a good option... not as bad as people make out (as long as you can play the class to your advantage), but not good on its own. Necromancy is all about the minions and, frankly, you'll never have enough of them on hand for the effort you'll put into them.
Dread Necromancer is nice, but you lose alot of versatility and nasty tricks. Buffing your undead minions with spells like Improved Invisibility or many, many, others gives them an edge that they don't normally enjoy (99% of undead being cannon fodder at best). In short, even though they get more Undead under their thumb, more is not necessarily better. Plus, they take all the disadvantages of Spontaneous Casting, most of which require you to spend feats that you won't be using to get the ones that make your minions tougher.
Cleric's a good all-round class, particularly if you've got living party members. Spell selection is wide and varied and you get almost as many spells per day as the Dread Necromancer. On top of that is armour proficiency and better BAB which is always useful. The key here is to cash in on the versatility of your spells to come up with more creative solutions to making the undead that you have tougher.
But, as I said, the whole Necromancy path is still deeply flawed, as many have pointed out. You'll never have enough minions, skeletons quickly become almost useless at higher levels (they are rather good at walking into traps...). The really good Undead are all intelligent, which is where you need to start looking at a high Cha character with maxed out Diplomacy, Buff and Intimidate. It may sound heretical, but if you can't control them with innate power, you have to convince them. Fortunately, Necromancy gives you an 'in' here, you can make life very painful for even Vampires between Rebuke, cure and other spells and abilities. Bargain with them, earn their respect, and carry a large smiting stick. It might be a pain in the butt, but it can be worth it.
Ok, I've ranted enough. DN or Cleric is up to you, I personally prefer Cleric but DN's a good class. DN you've gotta stick with for it to be worth your while through all 20 levels, Cleric gives you more breathing room... but you've still gotta keep your Rebuke and Caster Level up, so it can be a tad limiting. PrCs dealing with the Undead tend to be bad unless you can find an angle to use with one.
