Well, I'm currently using push contacts/calendar with manual email (gmail). I do also have push mobileme enabled on the iPhone, but I haven't given out that address to anyone and so push really isn't being used. I haven't noticed any significant battery difference with this setup (this is with a version 1 iPhone, and so I'm not using 3G). However, I always recharge the iPhone every night. I thought about forwarding gmail to mobileme, but I don't think I want that kind of interruption (besides, the lack of automatic incoming filtering in mobileme is a huge problem if you forward email).Pulling...ideally what I would hope for is manual/fetch email with my gmail forwarded to mobileme and then push contacts/calender. I would assume that since I do not use those two on a day to day basis it might represent a minimal loss of battery life as opposed to using with the push email.
Note that "push contacts/calendar" doesn't appear to be push; it appears to be more like "update when used".