Your question is like what's a better drink - milk or coffee. Some prefer the bitterness of coffee and the caffeine, others prefer the smoothness and refreshment of milk. Which one is better? Ask two people you likely get two different responses.
I've got a 60 Assassin and a 55 Necro, and I love em both - differently. Balancing life and death with the Necro is a real challenge at higher levels versus difficult content, but the Assassin (and Brigands) have their own strategic nuances to work on. They're both fun to play.