Necromancer:
Can be played as any caster (be a Ritualist or Elementalist and use your primary attribute for mana)
Awesome damage output, especially in hard mode and elite missions
Every party wants one
Pro Ranger:
Can play any physical class (use your primary attribute to have tons of mana and spam Warrior or Paragon skills)
One of the best utility classes in the game
Awesome caster control (BHA)
Fewer possibilities for high damage then Necros - but Splinter/Barrage is extremely strong
Builds for elite missions rarely use Rangers
If you want to do hard mode vanquishing or elite missions, I'd go with the Necromancer. But be aware that you might be used for spamming [[Blood is Power], which is about the most boring thing ever. But it still beats not finding a group with the Ranger

If you either have a good guild for elite missions (so finding a group is not an issue) or if you just want to play the game on normal mode, I'd recommend the Ranger. I also have a Necromancer and a Ranger and I think the Ranger simply is more fun. The Ranger can deal awesome damage and still has interrupts and daze and the possibility to play like a Warrior/Paragon.