Will I be able to revert to El Capitain if I install Sierra on my 2015 Macbook Pro? I have a 2015 (purchased in late 2016) Macbook Pro with the following specs : 


2.5Ghz Core i5
8GB of RAM
Running OSX El Capitain 10.11.6


It downloaded Sierra and is asking me to upgrade to the new version. 

I want to do it, but I'm worried that the new Operating System will be too demanding for my laptop or that there are some things that I won't like, and I might want to revert in the future.

If I install Sierra, will I still be able to restore my Macbook to factory settings (including El Capitain) by doing a factory restore (where you hold CMD+R while it's booting, and format the drive to reinstall the OS) or will it always default to Sierra instead?
Yes, after completely formatting your hard drive and booting in recovery mode from the internet you will be able to install OS version it was shipping with. Also, you can always download and install previous macOS versions from your App Store purchases.