What year Omnium are you looking at? Giant's geometry seems to change a lot from model to model.
I ride a medium 2008 Omnium (53.5cm tt), but I usually ride 53 or 54cm frames anyway. The compact geo on that model means the seat tube is only 50cm and you will have a lot of seatpost on show if you are used to riding 58cm seat tube.
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What year Omnium are you looking at? Giant's geometry seems to change a lot from model to model.
I ride a medium 2008 Omnium (53.5cm tt), but I usually ride 53 or 54cm frames anyway. The compact geo on that model means the seat tube is only 50cm and you will have a lot of seatpost on show if you are used to riding 58cm seat tube.
This is how much seatpost is showing on mine.