To warn you, it's no better on wp7 but I still doubt exchange is really killing your phone unless you live in a low signal area and you're constantly on 1-2 bars (radio will use more battery in this scenario). Push is not great but not bad - it opens up a persistent connection with a huge timeout, that's all it really is. Bigger things that eat battery tend to be things like face book sync (widgets especially that include pics) and the screen.
If you honestly want the best mail device it's still a BB with BES on the backend. The 9900 is bbOS done right.