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Quite a few people running in the range of 38-18 or 38-19 or thereabouts. Some people have used lower gears on hilly courses. Will depend on how you like to pedal, how strong you are, flatness of the course, mud, etc.
In general I reckon you're better off with the lower end for everything bar the dry flat grass crits. Even in those the dead turns, lumps etc will mean a slightly lower gear is that bit better for acceleration.
In the mud you can forget about anything other than grinding the whole way round on anything remotely not downhill...
53-11