I am currently looking in to purchasing an 8 bay NAS and am considering the following options:

QNAP TS 870 Pro
QNAP TS 869 Pro
Synology DS1813+

The QNAP TS 870 Pro has a better processor than the other two (Intel i3 3.3GHz compared with an Intel Atom 2.13GHz) is the performance increase between the two likely to result in a noticeable performance gain in real world usage? we are most likely going to be storing all our various videos on this NAS, how many of these are we likely to be able to play at the same time before we start to see performance issues?

Do both the QNAP and Synology systems both have a volume shadow copy style function to allow the ability to go back to an earlier version of a file or recover the file if it is deleted? (I believe this could be acomplished by connecting the NAS to a server using ISCSI and actually use volume shadow copy but I am not sure we will be going this route at this time).