In my experience it depends somewhat on the sites/web apps one uses (some are not well optimized for WebKit), but Safari definitely tends to be easier on the battery. It seems to try harder to get to an idle state and keep CPU usage down where Chrome and Firefox are happy to keep the CPU spun up, perhaps due to a “speed and bells and whistles at all costs” mentality in development (traditionally browsers have been marketed on speed and features rather than efficiency).