I have the 14mm plates that fit a standard Dynafit Radical or classic heel pattern for use with my Kreuspitze heels and Radical toes. They are simple and impressively light, thanks to strategic material removal and thin backing nuts which minimize plate thickness and ramp angle change. 14mm is plenty of range for my collection of boots (with a little forethought in mounting - consider the 6mm gap spec when doing so).
The mass penalty for the 18mm or 40mm plate is very small, but I have a near-perverse insistence that all of my skis have inserts and the same patterns so I can swap parts between them at will. If you have no such requirements, consider the longer plates.
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