The disadvantage of using pre-compiler commands is that some IDEs (Xcode specifically, perhaps they all do this) don't color disabled code any differently, so at a glance you can't tell that the code block won't run.

Before a commit I'll sweep through and delete the intermediate code blocks that I've commented out. If I use pre-compiler flags to comment out the code, it's easy to miss it in the sweep. Because of this I never use the pre-compiler flags for something I don't want to get committed.