Great question — answering as the UphillTech team:
This is not about gearing. You choose the gear that suits your effort on either wheel size. What mainly differs is the ability of the wheels to roll over various track unevenesses, stones, tree roots, dips and so on. The larger wheels simply run over these much better and the smaller wheels get easily stopped by some roughness. Small wheels can be more agile when you need sharp turns, but whether it will save you any time on any particular course is hard to say and probably can only be estimated by trying both.
Full answer by Vladimir F Героям слава: https://bicycles.stackexchange.com/a/97340 (CC BY-SA 4.0).
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