The two devices you mentioned are probably the best
Android boxes on the market
today. However, neither are perfect. I cannot say which one is "better" since I only have real experience with one of the devices. Worth mentioning is that new devices with technically better hardware (the more powerful Amlogic S802 chips) will be released very soon, such as the MINIX Neo X8 and Neo X8s.
I myself have a G-Box Midnight MX2. I was initially disappointed with it, but after installing the
MX Linux firmware, I am pretty satisfied. It is not without flaws however. In fact, there is no shortage of problems. For example, the box currently cannot smoothly play very high bitrate video (seems to have trouble on 20+ Mbps video from my small testing), and I also have a problem with FF/RW (wherein while rewinding or fast-forwarding the on screen video does not sync up at all with the times it is being rewinded or fforwarded to). And those two are only the problems which bother me personally. There are many other problems with this device (it is worth noting that the Linux builds for this device anecdotally have much fewer problems in comparison to the Android builds). However, for my usage I am at the end of the day pretty happy with the device I got for this price point.
So the G-Box MX2 (from my experience) and Minix Neo X7 (from what i heard) work pretty good today, but not flawlessly. The new
very-soon-to-be-released Amlogic S802 will probably work better, but
may suffer from early problems these type of devices tend to suffer from in the early months after they launch. So the choice is yours.
However, you may also want to reconsider whether you really want an
Android box. For slightly cheaper you can get a Raspberry Pi, which is very well-supported and works very well with XBMC (the main disadvantage with the Pi is, from what I heard, menu navigation in XBMC is relatively slow). Or you can spend a little more and get a cheap Intel NUC box (such as the NUC DN2820) which will allow you to run XBMC better than any other cheap device. So the choice is yours, hope I helped.