Other than a perceived "need for speed", what is missing from 5.1 you are looking for? Android 5.1 is chip specific for the underlying processor S905 and the S905X uses Android 6.0
here is a good reference, https://www.cnx-software.com/2016/07/31/amlogic-s905-vs-s905x-benchmarks-comparison/
and Quoting, "Please note that currently most S905 TV boxes run Android 5.1, while Amlogic S905X devices are already running Android 6.0, so the latter may benefit from some small performance boosts due to more recent firmware. But in any case, that represents a side-by-side comparison of what you can expect from devices sold on July/August 2016, although some Amlogic S905 devices are now getting Android 6.0 Marshmallow firmware updates.
The main takeaway is that you should expect slightly lower performance from Amlogic S905X compared to S905, so the new processor is only interesting if you need 4K VP9 hardware decoding (not usable for YouTube 4K, except in the upcoming Xiaomi Mi Box), and/or High Dynamic Range support, or if the price is cheaper compared to an equivalent S905 TV box."
If anything the ATV 495 Pro has double the memory @ 16 GB eMMC of the ATV 495Pro HDR
Hope this helps