I heard* that for encrypted video playback (i.e. DRM) the Intel GPU/i5s were outperforming Nvidia/ARM tablets by a full hour of full-screen playback in terms of battery life. That was a long while ago (like 18 months ago), so it might have changed since then.
For normal computing the i5 will obviously lose out - so you'll have less time with your Office 2013 on Pro than on RT, but for Netflix (which is DRM fullscreen video playback) you might see it swing the other way.
* a long time ago in a galaxy far far away. Terms and conditions apply. Not official Microsoft anything.