10.4 vs 11.x

First of all I suppose is the date of release.
11.1 will be released around 26th of July 2017. https://www.freebsd.org/releases/11.1R/schedule.html
10.4 will be released around 3rd of October 2017. https://www.freebsd.org/releases/10.4R/schedule.html

Secondly, please look at the installation/upgrading instructions. There are some different techniques to do that.
11.x https://www.freebsd.org/releases/11.0R/installation.html
10.x https://www.freebsd.org/releases/10.3R/installation.html

Thirdly, in my opinion, 11.x suports a lot of more hardwares than 10.x .

Fourthly, look at the releases notes and compare each other.

Basically, somebody else could provide more practical cases.
Even like me, I used 10.x before and I currently use 11.x, I cannot better describe differences, instead above links.