It really depends on the amount of music or video files you have. I chose the Q5 simply because I run a system of NAS drives which enables me to watch movies on any of the 3 TVs I have. As I have over 5,000 movies and 32,000 music tracks a single HD would not do the job.
Use a 4-port USB docking for my movies and an Internal HDD for pictures and music, this setup also dupes as a combined NAS for all other media players around the house. Works just fine for me. In fact sold my 2-slot NAS again. The dedicated NAS alternative would mean an 8-slot high end NAS which is very expensive, eats a lot of energy and worse a NAS only allows identical HDD for the whole lot; even in JBOD mode it does not allow Plug & Play. Now I have a true Plug & Play setup (even Internal and USB Docking HDD's are fully interchangeable). Just pop-in the drive you like, no need to power-off Q10 Pro (using eject). RAID is too expensive for me, already spending a lot on HDD's.
For uploading content use FTP (not Samba) which is fast enough.
The only disadvantages are that NAS obviously shows up as 5 individual HDD's and you can't setup individual access rights per user as a real NAS would facilitate.