A 250w hub is fine for level ground and mild inclines, but anemic on hills. A 350w geared hub is not much larger in diameter, but noticeably stronger. Like a 250, it can also be used on steel forks without a torque arm. I had bikes with both sizes and compared them on the same climb. Where the 250 need to be zig-zagged to maintain road speed and keep the motor from bogging, the 350 would go straight up.

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Leed PBJ minimalist system
Clean Republic 350-watt hub review