The reason why apple decided to implement only one button on the iPhone since the very start is to give the user a fallback solution. One can always return to a familiar scenario if one feels lost. This is crucial to a big number of users and maybe not to so many (like us discussing this ;-) who think about function and technology behind things. All the extra features like double tapping etc. are exactly for us. Most of my none technical aware friends don't even know of the existence of these extras. I've been using an android phone once or twice up to now but never really understood what app would justify the existence of a separate back button (which couldnt have been implemented per software). Maybe my apple influence is just to big here so I'd say both solutions might be equaly good. One likes to go back step by step, the next directly back to the home screen. I suppose there's no difference.