My road race bike and my TT bike are mens and womens, and not the same sizes exactly (I'd have to go measure cause I can't remember, and its 6:30am and cold and dark outside...)
But I would suggest you sit on them, get measured and fitted for the 50cm as well and then go for a ride on it so you can compare with the 54cm...
My TT bike is smaller because of the position I ride in... but you are right - a 4cm difference in frame size is significant...



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