Depends on your budget limitations and type of grinder.
The Mignon and Rocky can be compared, they both use 50mm plates. The method of adjustment of the mIgnon is superior to that of the rocky. But at the relative RRPs it costs $100.00 more than the rocky. Is it worth $100.00 more at RRP ? I think so, but you mightnt.
The K3 has from memory 58 mm plates and thefore competes with Mini mazzer and Macap M4 rather than the above 2. So its in a different "class".
Is there a real difference in the quality of grinds between a 50 mm grinder and a 58 mm grinder? Is the extra price tag justified?
Perhaps that will be the subject of a future test at my place!