In reply to matt1024:
Doubles-
Pros-
Less drag on wandering pitches.
Ability to make full length abseils.
Redundancy if one rope got severed.
Cons-
More faff.
Heavier (usually).
Can increase impact force on gear.
More expensive (generally).
Singles-
Pros-
Much easier to use.
More resistant to running over edges.
Can have less impact force on gear.
Lighter (generally).
Cheaper (generally).
Cons-
Rope drag on wandering routes.
Heavier for one person to carry (you can take a double each to spread weight).
Can only abseil half rope lengths (or use a retrieval cord-faff IMHO).
If you run out of rope half way up a route, you can't get creative and get a vital few extra meters out of the system (not recommended or a regular problem).