I'm not an expert on bike geometery. My limited understanding, however, is that the head and seat tube angles will have an impact on how "relaxed" or upright the riding position, as well as stem length, etc. I do know that if there is even a slight difference between the bikes in how upright you are riding, different seats make sense. The more upright, the wider or more flat the seat; the more aero the position, the more narrow and/or rounded the curve of the seat.