It's not a matter of "a better army". But more of a "which army do I like the feel/look of?".
Dark Eldar are [supposed] to look dark, evil, and brooding, and are extremely fast on the tabletop. They are, however, very fragile, and don't have too much in the way of variety. So stealth and speed are the trick with these guys, you have to be VERY strategic. Used correctly though, they will decimate your foes. Also, new models (along with a new codex) are supposedly in the works right now, so you may want to wait for that.
Now much the same could be said for Eldar in gameplay terms, but I should note that EVERY unit in an Eldar army is specialised. I.E. Howling Banshees are good at taking down small, but powerfull groups, such as
SM, or
CSM, Striking Scorpions are good at taking down hordes of weaker troops, Dark Reapers are good with long ranged combat. And every unit needs to be used for their specific role to support each other, or they will crumble. Again, with the exception of a few units, are very fast and fragile.
Now that brings me to Necrons. I don't know too much about them, but here's what I know. They are slow, but incredibly powerful and hardy. And due to their "I'll be back" rule, they are very survivable. Also, they have a high Toughness and Save. So yeah, they're slow and powerful. Though I've heard they're not too good in melee.
And there you have it.
Hope this helps