The Realtek boxes all produce a better image than any Amlogic. Deeper colour and a more movie like sharpness. It's like the difference between a good BD player and playing a movie using a Blu-Ray drive in a PC.
The 1619DR chipset in particular produces the best image of any Android based (i.e. in last 10 years) media chipset, on a par with HiSilicon and similar to the very old Sigma Designs boxes (Popcorn Hour, original Dunes).
That's the reason why all the high-end Media Player brands use Realtek and not Amlogic.
As to what to buy, Zidoo use Realtek 1619DR and they have the best firmware (excellent media browsing software and few bugs). They also have a busy support forum, responding to customer feedback with a steady stream of updates. Dune also use Realtek 1619DR and, whilst I appreciate some people prefer their software (not Android and for legacy reasons), their media browsing software is inferior and the firmware is less reliable with fewer options. The new Zapittis use Realtek 1619DR and have good media browsing software, but it's up to the customer whether this justifies the extreme pricing.