Official Gmail app has push. I find the much better integrated with the actual Gmail inbox environment than the default Mail app on the iPhone. => for me no reason not to use Gmail app especially if I want Push.
On the other hand, I'd say the Gmail app is a bit less integrated with the iPhone as the default Mail app is - i.e. for "mailto:" links, there is no way to set the Gmail app to open and start composing a message. Hopefully this would change in a future iOS version.
(remember when the Facebook app was equally as non-integrated before - Twitter being the only way to share content)
On the other hand, I'd say the Gmail app is a bit less integrated with the iPhone as the default Mail app is - i.e. for "mailto:" links, there is no way to set the Gmail app to open and start composing a message. Hopefully this would change in a future iOS version.
(remember when the Facebook app was equally as non-integrated before - Twitter being the only way to share content)