It's all about preference. Older tutorials (before storyboards were introduced) will tell you to use XIB files, however, storyboard can be used as a complete replacement for XIBs.
If you prefer using XIBs, you can choose to stick with them. A storyboard is simply like a bunch of XIBs on the same page, which eliminates the number of files you will have in your project.
Using a storyboard will change some of the methods that you need to use in a View Controller class -- such as
-(id)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil