Let me share my experience with all users/potential users of Q16 quadcore, especially for those who have used Q5ii dualcore.

My review of Q16 quadcore (vs Q5ii dualcore)

Background
Used Q5ii dual for a year, Q16 quad for a few days.
Unifi VIP10
Asus RT N12HP router, ethernet and wifi
Static IP
1-7 devices used at any one time
Double storey house

Startup/Shutdown
Startup and shutdown is just as fast if not faster than Q5ii (emmc).

Launcher
If you don’t like the original Mango launcher, you can install another launcher like Apex or Nova. If you wish to go back to Mango launcher, just press home. Then, after programming a fast key to access your preferred launcher with the new shortcut app, you can toggle between the 2 launchers.

Flash memory
A big memory is necessary to install and run more and bigger apps. Apps like XBMC and Astro-on-the-Go take up a lot of memory. For Q5ii, it is not possible to play 1080p files in Popcorn Time due to its small memory. In the case where you have little memory left after installing many apps, even 720p files won’t play, so you need to clean cache or remove 1 or 2 apps. With Q16, there is ample memory for many apps such that you need not worry about constantly cleaning cache or removing apps. In numbers, you may have a situation like 300MB available on Q5ii but 3GB on Q16 - for the same apps.

Android 4.4 KitKat/1080p kernel vs 4.2 Jellybean/720p kernel
I do not know if it is due to the android version or the kernel but I can safely say the video output from Q16 is a touch superior to Q5ii. The colors are more vivid, definitely deeper for USB playback, likely due to a new feature called “photo quality setup”. If you find the colors too deep, use menu button to access that function, play around with the options including “user defined” where you can reduce hue and saturation.

China Apps
With same settings, I find streaming from Chinese sites Pudding, VST, PPTV and IMGOTV (my ranking in that order) smoother and with improved chances of getting the highest resolution available (“blue ray” or “super”). Q5ii works well on these too but more likely can only get “standard” or “high” resolution and not “super” most of the time.

XBMC 13.2 Gotham
There is not much difference that I noticed. Except that in system/settings/video, highest resolution setting available is 1080p for Q16 while it is 720p for Q5ii.

Popcorn Time 2.3.2
This is my favourite movies/TV series app. Q5ii can’t play 1080p files due to memory limitations, can play only 720p at best. Q16 can handle nicely all that is available and working.

HD Audio
With audio pass through, BDMV/ISO/m2ts work flawlessly delivering DTSHDMA audio and enabling full menus – same as Q5ii.

Samba
Likely due to improved and bigger resourses of the Q16, the Samba function works much better than in Q5ii. As host, Q16 transmits via wifi BDMV video and DTSHDMA audio smoothly to the Q5ii but not the other way round. Q16 can receive DTSHDMA smoothly from a PC but Q5ii can’t. Q5ii can handle up to DTS only for the most part. I have not tested the Samba 3D, UPNP and NFS functions.
On Q5ii, auto search on LAN does not work. On Q16 quad, it works the way it should - auto detect your PC and other powered on devices on same LAN to seek to access file sharing. Just key in id and pw, save the shortcut when prompted and it’ll get to work.

Not-so-good
At this time, I really have not found any serious flaw or problem that deserves mention here. But I do want to comment on a related item.
Mele F10 airmouse
Not a Himedia product, the Mele F10 airmouse do me no favors. Probably a result of using normal mouse and Himedia remote control for so long, it is hard to adapt to some of the unconventional strokes with this airmouse, the major one being:  on an ordinary mouse, there is left click and right click, a roller in the center to scroll up/down.; on Himedia remote control, the 4 arrows up/down/left/right surround the OK/enter button – all that is good and normal. Now, on that Mele airmouse, the center button with 4 arrows around it is not “OK/enter” but is left click! While the left click button is not left click but is “OK/enter” – i.e., the buttons are “reversed”, so you make mistakes all the time, naturally. And that is annoying. A case of poor design not following conventions and intuition found on other common control devices?
To scroll, you must get out of mouse function (pressing another button twice is a pain), to use arrows. Only one function can be used at one time – either as mouse or as remote control.
The keyboard buttons are small – too small for my fat fingers! It also seems too sensitive, need two hands to steady it esp to click a small spot.
This airmouse work with Q16 but not recommended for Q5ii. With Q5ii, the lower functions like arrows, enter and back are fine but using other functions like menu can cause the device to crash and reboot after which the wifi function is turned off - probably an unexpected shutdown security measure.
Unless you have tested and liked this airmouse, my advice: better get a wireless mouse.

My Conclusion
I say it is worth paying a bit more for Q16 quad rather than Q5ii dual (can’t say anything about Q5ii quad). I understand there is the issue of VPN not fixed yet for quads, hopefully it will be resolved soon. And hope that a later firmware version will fix some of the kinks known so far.
Lastly, a whole lotta thanks to Nicety who has continuously proven himself to be a top notch online seller as being patient, conscientious, knowledgeable, quick and helpful to us customers and users.
My comments are based on my own experience; if any user has a different experience and that is always possible given the use of different isp’s, broadband speed, routers, etc., please do not hesitate to share. It is the sharing of experience that allows us to derive maximum benefits for a product we acquire. I have benefitted from many users here and always willing to share with others anytime.