Personally depending on what your aim is i would just say buy something that is being actively developed for what you want. If your going to use Android a lot then you want someone that actively supports the firmware for that box, If your looking to make the use of LiberELEC or similar then make sure what you buy is being supported by one of the developers here or one of LibreELECs forks.
Run LibreELEC off the sd card.
$25 extra for DDR4 is only worth it if your gonna go about making sure the firmware timing is proper to take advantage of it and even then the gains may only be marginal.
Just keep in mind that no matter what any dealer says if the box has a S912 in it then they are all basically in the same boat no matter what. so just make sure whatever you buy that someone is actually supporting.