Depends on how many emails you usually get. (Or whatever it is you're pushing and fetching.)
If you get a lot (like, let's say 15 an hour) then fetching will use less power. If you let less (like, 1 every 2 hours) then pushing will be better.
In between? Hard to say, you'll probably have to push for a week and then fetch for a week and compare the weeks. We really have no way of answering you since everyone's usage is different.