Booxstor firmware is also official Onyx firmware. Usually what goes to "Booxstor firmware" shows also in Arta Tech one, but maybe several months later.
Also note that difference is not 70 EUR as you have to buy also good cover and maybe also power adapter if you do not have any.
And sooner or later android might be ported to M92. Booxstor mentioned it might be problem port android also to Arta Tech.
And there is probably difference in Warranty lenght. Booxstor has 2 years, ArtaTech 1 year.
My opinion is that there is not very big price difference if you take in account you have to buy a good cover and power adapter and the risk something breakes the 2nd year...
So, the choice is not the cost, but if you want better TTS at the cost of delayed firmware updates and the potential risk android will not be ported to device.