Here is a link to Cube Mix Plus forum post about the sale: https://techtablets.com/forum/topic/gearbest-flashsale-cube-mix-plus-for-280e/

Granted, there isn’t a huge difference between the tablets, but off the top of my head I can think of a couple of good reasons to make the switch now:

1. When the price of the M+ was $340-395 only just recently, upgrading might not have seeemed like a good idea, but @$299 the overall cost to upgrade might now be worth it.
2. If you were planning to open up your I7b to do the thermal mod or put a bigger SSD in, know that even though the M+ still could use the thermal mod, it already comes with a bigger 128GB drive, so that might be one less thing to do. If I had put off modding my I7b until now and was on the fence about an upgrade, I think I’d rather crack open the model I had a better chance of continuing to carry around in another year or two, it’s exactly the same procedure.
3. Other than a color change, the only other notable difference is the Kaby Lake processor, which is bit faster and most importantly has built-in h.265 support.  Much better on battery life if you regularly watch h.265 video, and although I haven’t tested it myself yet, it might even make editing short 4k h.265 videos possible/tolerable.
4. Other than color matching, there is no need to upgrade your keyboard, as the M+’s kb is exactly the same model in silver.  (and of course you could keep your pen too)

Honestly, if I had an i7 book setup the way I wanted and didn’t care about the bigger drive or processor upgrade advantages, it think it would definitely be easier to keep the I7b, as it looks to be almost exactly the same otherwise.

But maybe the good upgrade price is decent reason to indulge in a little future-proofing, and in any case I’m posting the $299 sale link in this sub-forum because there is lot of good info posted here by mostly happy owners of the I7b, (OSX Sierra?  Wow.  I can’t wait to try that when I have the time) and as more people starting getting the M+,  I believe that sub-forum has a chance at getting as interesting as this one.