Theoretically I guess, push should save you battery life compared to fetching if you make less changes to your contacts / calendar than the number of fetches in a day.
If you think about it, if you set it to 15 minute fetch. It'll check your contacts / calendar 4 x per hour x 24 = 96 times a day, whether there are any updates or not.
With push it'll only update whenever a change is made. If that's none in the day or less than 96, then I'd assume it'll save you a fair amount of battery life.
I'd assume the reason people are experiencing lower battery life with push turned on is that you've still got Gmail fetching every 15 minutes (as it doesn't support push on the iPhone) and you're playing around with contacts / calendar a lot (partly with it being new and to see how well it works) which is forcing a lot of push updates. Between the two that'll drain your battery life quite quickly. (Remember every minor calendar / contact change will normally trigger a separate update). When they bring out push Gmail for the iPhone, which I assume they will soon enough, that should go someway to helping.