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On the DIN chart I looked at, even as a level 3+ skier your rec. DIN is ~7. Honestly I think a 14 DIN would be fine, unless your planning on running your DINs below minimum; but if you're not even on the DIN range, you probably don't need them.
Taking your DIN setting out of the equation, all you'd gain by going with an 18 is the more durable metal PivotRace Toe over the plastic Dual Action III toe on the 14.