Both phones should get you through a day of usage. I need to get my hands on one to try it out but right now with my 7 Plus I have 29% battery left and have got 7 hours 56 minutes of usage (SoT). I have been off from grad school for fall break and been on the forum, youtube, and internet researching the Pixel XL. People need them longer than a day or 2 to really know how they will do but I would expect the Pixel XL to be close to the iP7+.
Always gotta have pictures to back up my claim for my iP7+ though. Lol