With the upcoming release of both Amlogic S912 and Realtek RTD1295 I think it will be useful to compare them, along with the already available HiSilicon Hi3798C V200.

We already have a bunch of articles about HiSilicon Hi3798C V200, and we did review the best device (so far) that uses this chipset (Himedia Q10 Pro), so we know quite a bit about this chipset's capabilities. Simply put, the V200 is currently among the best available SoC for TV boxes (the only real competitor is the Nvidia Shield).
There are just a few manufacturers that launched devices with this SoC and HiMedia is the one that gets the most attention due to their efforts to solve the unavoidable bugs via constant updates. Himedia are working closely to the engineers at HiSilicon for this and so far the results are showing.
Being the only SoC that was released already, the V200 does have the upper hand - most bugs are fixed, the specs have been verified by numerous reviewers and the overall stability of the devices is known.

I first heard about Realtek's plans to release a new TV-box-oriented SoC almost one year ago, so Realtek took their time to release the RTD1295.
As I mentioned in the dedicated article, Realtek plans to gain some of the lost market share with this SoC (they ruled the media players niche a few years ago) and my initial impression is that they are trying a bit too much: RTD1295 has HDMI-in recording, can act as a router - it's very impressive on paper and delivering all those promises might prove to be difficult.... but they seem to be on the right track.
If HiSilicon has Himedia, Realtek has Zidoo as the main manufacturer - and they are doing a great job so far promoting the SoC (and their devices), with many teasers and good social network presence.
Zidoo X9S was recently put up online (presale) and most of the features Realtek announced for their chipset are available on this device. Zidoo even published some pictures with their engineers working closely to Realtek's so we know they mean business.

Last but not least is the Amlogic S912 SoC. Amlogic managed to get much more manufacturers involved and the prices are much more appealing than for the other two (lowest price so far is around $50).
But unless Minix will announce a new S912 device at IFA Berlin next month (and I hope they will), the SoC lacks a manufacturer that stands out from the crowd with after-sale support the way Zidoo or Himedia do for the other two devices - there are many brand-less devices with this SoC and just a few brands we heard of (RKM, Nexbox, to name a few).
But let's not hold that against them and give the SoC a chance, as it most certainly does look very good on paper - in fact, if we rank the three SoC's solely from the specs per price this one will surely be the winner.

The comparison below is work-in-progress, it will be updated as soon as we'll test the new devices (Zidoo's X9S will arrive for review soon).

Looks like we're going to have some interesting devices this fall, and the competition for the high-end market will help interested parties such as yourself to get a better price. In fact, if you're looking for one of these devices just email/tweet and we'll see if we can help you out with a coupon or a better price directly from the manufacturers or from our friends at the online stores.