Hi, I have already Tried both of these Operating systems, they were both Good when it comes to features especially the nougat one, nougat has this secret thing when you press the setting/gear icon for about three seconds it will activate an option similar to God mode on computers, I will base my answer on how i handle my computers, When the hardware is already strong you can choose to use the updated operating system but for me I prefer to use the outdated one because it consumes just a small amount of your hardware's performance/Power, it doesn't have to many system requirements, you can Still Squeeze the true potential of your Computer's Hardware.
7.0 Nougat is the best. I like about it is you can easily adjust your notification settings whenever unimportant notification alerts suddenly appears just slide right the notification and choices will appear.
The best and most useful feature of Nougat for me is the Split Screen feature in which you could open 2 apps in the same time. I can now open Facebook and YouTube at the same time.
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: