A question on the Ray Wenderlich forums peeked my interest. It was wether a beginner should use Xcode 6 or stick to Xcode 5. The main raison for wanting to stay with XC5 would be that it match screen-for-screen and feature-for-feature with tutorials / books.
My advice would be to use Xcode6 regardless of this. Most of the difficulty you would face is in tools like interface builder. Since especially interface builder has become a lot better it would be more beneficial to read up on the differences and use the XC6 version.
The SDK has also been updated in significant ways. By using version XC6 will also immediatly get deprecation warnings. These help you understand that the things your book / tutorial teach are part of "the old way" of doing things.