Both armies will give you a good chance to win vs many armies. Unless the army is particularly bad, you will find that generalship is more important in win/loss ratio.
As for the two armies you picked out: skaven have a number of competitive builds, but lets be honest, they lend themselves to a horde army just on how bad the basic guy is, and how cheap you are getting him. They have a number of very interesting units, wacky special rules, and some things some would call

Certainly entertaining, but personally not my choice.
Dark elves are a new book, and have what some people call codex creep. I call it they don't suck anymore. Just about everything in the book is good. They are good/competent in all phases of the game. The biggest weakness is durability, T3 with underwhelming armor doesn't last long. They have a good assortment of monster type creatures, and there is a large number of effective builds that can be built out them. Just off the top my head they can field the typical
MSU army, a rank and file type army, a fast mounted army, the monster heavy army, the chariot army (8 possible chariots pulled by cold ones!), and the magic and shooty army. Their rules for magic and the power of darkness spell can let them clean up in the magic phase if you really want to. If your group allows special characters,
DE have some really good ones.