I'm not sure what I make of most of the battery saving recommendations. I read conflicting info all over the place. I'm willing to sacrifice some battery life anyway to make my smartphone a little "smarter".
To grossly oversimplify it, "push" very roughly works by the same mechanism that alerts the phone when a call is coming in. To be more specific, the phone leaves a data connection to the server hanging open but doesn't actually transmit down it, so no power is consumed. When the server sends, down that socket to the phone, the notification that an email has been received, the phone receives that notification via the same receiver used to detect incoming calls. If you use the timed interval email checks vs the "push" option, the phone's transmitter has to fire up every X minutes, and this consumes battery. Think of a phone's talk time vs standby time to get an idea of how battery performance comes into effect.
The reason it might chew battery for some people would be if you receive an absurd amount of email, and the phone is always firing up to receive them. Because the push is just a notification that an email has to be fetched, it's not the email itself. The phone has to fire up the transmitter to receive the email. So, if you get 5 emails a day, push is going to save you battery. If you get 300 a day, your transmitter will fire up 300 times, vs the ~100 it would if your phone was grabbing them every 15 mins.
Also, to enable "push" on gmail for people who don't have it, go to the phone's inbox, press (...) then settings. Click on "sync settings". Enable calendar sync and then disable it. The act of enabling either calendar or contact sync, just for a minute, will switch the entire account into a different communication mode. Then you will see under "download new content" a new option "as items arrive". That is the push option.