Ming-Chi Kuo, the famous KGI Security analyst, is now claiming that Apple's iPhone 6 5.5" model will be delayed because of constraints with some key components. He claims that current supply chain contacts say it will be pushed post October (possibly into 2015).
He claims the issue may lie in the drop test durability of the new sapphire display. His "unnamed sources" claim that when the larger iPhone 6 is finally released, fewer than 20 million units will be available.
The 4.7" iPhone 6 is expected to have the same issues but he believes solving it on a smaller display size device is much easier. he also highlights the fact that there have been more leaks of the 4.7" iphone 6 than the larger 5.5" one.
The truth is that Apple hasn't announced anything yet and I don't think it is fair to say an unannounced product is delayed. It can't be delayed until Apple officially provides a release date which is missed. So take all of this with a grain of salt.
If you need a new iPhone then get whatever they release in Sept-Oct but remember that most generation 1 releases have issues that are worked out post-release. I prefer to stick with the "s" iteration of the iPhone's to ensure most kinks are worked.