The 850 is air and the Emax is a micromotor. The Emax as much greater torque but the 850 makes up for it with blazing speed (400,000 rpm). Both will work perfectly for background removal in relief engraving, but I prefer the Emax because I can feel the grinding action better than with the 850.
If you plan to do stone setting, the 850 is not what you want for preparing stone seats.
The 850 consumes a lot of air so if you have a small compressor it will probably run continuously as you use the tool, and possibly overheat.
If you get the Emax for removing backgrounds, be sure to buy the 1/16" collet so you can use dental burs.
It seems the burs last longer with the micro motor too. They are spinning slower so there is less heat and you can usually get more use out of each burr (at least for me).