GCD is a lightweight way to represent units of work that are going to
be executed concurrently. You don’t schedule these units of work; the system
takes care of scheduling for you. Adding dependency among blocks can be a
headache. Canceling or suspending a block creates extra work for you as a
developer!
NSOperation and NSOperationQueue add a
little extra overhead compared to GCD, but you can add dependency among various
operations. You can re-use operations, cancel or suspend them. NSOperation is
compatible with Key-Value Observation (KVO); for example, you can have an
NSOperation start running by listening to NSNotificationCenter.
原文：http://www.cnblogs.com/wangrui-techbolg/p/3567450.html