Amlogic S905X is hot at the moment, and it is said to be released in a few weeks.
S905X is a cost effective model based on S905, the greatest spotlight is that S905X supports VP9 decoding while S905 not.
(Both of them support H.265, Click to see H.265 vs VP9, which one is better for 4K video)
Another one is the price. As a newer model, S905X has better performance, however, the price is same or a little lower.
To be simple, we conclude it in a chart:
Check more differences between the 2 models: