I think battery life will most definitely be worse on iOS7, due to, of all things, the new design.
iOS 6 uses a lot of darker colours compared to iOS7, and iOS7 is almost completely white.
Using some physics knowledge, I can say white is the most battery draining colour to display on a screen, because every wavelength of visible light must be emitted to produce white light. More wavelengths = more energy needed to produce light, so that draws more power from the battery. In addition, blue light has a higher energy than red light (e=hf), so if you want to save as much power as possible, pick a dark red wallpaper!
Not a very power efficient design choice from Apple this time... Or you could invert the colours of the display in Accessibility settings, which would improve battery performance, in theory. And when, for the average smartphone, 70% of the power is being used by the display, doing what you can to reduce its power consumption helps greatly.