Push E-Mail will only transfer a very small information header and the e-mail itself. Fetching E-Mail, on the other hand, will transfer handshake info, your login name, password etc. every single time the iPhone fetches the messages. Also, push e-mail via mobile me is compressed which reduces data size - while IMAP fetching isn't. Each time your client fetches E-Mails, it transports 10 times more data than the amount of a push e-mail.
If you set your fetch intervals to an hour then yes - the battery will probably last longer. Anything below that may hit the battery even worse than push e-mail