Personally I'd go with a Necro just because of their soloing capabilities, but it really comes down to personal preference.

Pros
 

Iksar Necro Pros - You're an Iksar Necro

Iksar Necro Cons - Lmao....

Cleric Pros - .......

Cleric Cons - You're a Cleric.....

Human Clerics can get some nice toys, follow innoruuk, and dont level in crushbone, maybe even try the warrens, you get a clicker snare necklace, fear clicker mask, instant junk buff see invis helm ( great for raids later) and a bad arse purple skull shield for the early pre 55 levels.

Why not both? I have an epic cleric and epic necro, love them equally.

Go Cleric.

For no other reason than Clerics are the "prettiest belles of the ball" in Velious. You basically have the golden key to get into nearly any guild you like, as Clerics are so vital. When you start to burn out, or if you even do, level up your Necro alt for funzies.

Pretty much what Vowels said.

Or Netted Kelp (droppable) or Flayed Paebala (nodrop ultra rare). Probably some others too.

I've decided to go with Cleric! I'm making a necromancer alt too, though. Maybe even an Iksar necro, considering how much I hate the leather/mail/plate graphics they have.