Definitely Nougat. It's faster and there lots of quick access features. The API, of course, gets higher, so more and better functionality. Its battery life has been improved. It lasts longer than Marshmallow. You can multitask because it has a split-screen feature. In Marshmallow, when you want to clear all your recent apps you have to swipe each tab one by one, while in Nougat there is a clear all option.
Here is a quick summary of the update:
If you want to dig deeper, here is a review comparison of Marshmallow and Nougat: