I look at it this way: they both work, but shackles are safer and you don't have to worry about your line slipping out of a hook.
With that being said, if you have no aftermarket bumpers, its easier to just slap some tow hooks onto your frame.
However, If you have an aftermarket bumper with shackle points welded through the body of the bumper (constructed correctly), then stick to shackles because there's no reason to use tow hooks!