The whole reason for push is so you can get notifications sent to the iphone from an app ex: AIM, without the iphone checking at all, push will barely have negative effects on battery life
Push will significantly decrease battery life on an iPhone, for email or anything else.
Take MS exchange on the iphone for example. Apple's push machanism is done at the software level. Therefore, there's always an open idle connection with the exchange server, constantly waiting for a new message. This open connection stays open, always, and therefore drains battery life like no other.
Fetch is far more convenient as it will maintain your battery. Simply put, after it checks for messages, it then closes the connection, unlike push.
Blackberry's are far more superior in terms push email technology. This is because push is done at the network layer, and not the software layer. IE: a new email comes and is forced to the phone, without the phone waiting for it. Think of it like a text message. Is your phone maintaining a constant connection with a 3rd party server? No, it's only connection is with ATT, and the text message is then pushed through their network, through the reception/signal connection. (technically data, but they don't consider it to be)
On another note, this is all coming from myself, an iPhone guru. I love this phone, but unfortunately it doesn't have very good push email.