dispatch_queue和NSOperationQueue有什么区别？

Question

我想了解dispath_queue和NSOperationQueue之间的区别，当我可以将我的代码编写为：

let queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)
    dispatch_async(queue) { () -> Void in

        let img1 = Downloader.downloadImageWithURL(imageURLs[0])
        dispatch_async(dispatch_get_main_queue(), {
            self.imageView1.image = img1
        })

    }

和as：

queue = NSOperationQueue()

queue.addOperationWithBlock { () -> Void in
    let img4 = Downloader.downloadImageWithURL(imageURLs[3])

    NSOperationQueue.mainQueue().addOperationWithBlock({
        self.imageView4.image = img4
    })

}

那么，这2个区块有什么区别？如果有可能，请为我描述一下