(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:
Recommended Models, No Regret to Buy:
MECOOL KIII PRO Hybrid DVB TV Box, 2018 Best Featured Model
Mecool M8S PRO L, 2018 Best-selling Model in EU
MECOOL M8S Pro+ TV Box, (Best Seller of Quad-core TV Box)