@cdvn - the best thing you can do is get a bike fit. Better still, get a 2nd opinion. Get 2 bike fits! Then you'll know the right frame size. I ended up getting a 52cm Addict R3 - very similar geo to the SuperSix. But when I considered the SuperSix 105 I was torn between the 50 and 52cm frames. My friend is 177cm, rides a 54cm CAAD9 (same geometry) and regrets not buying a 52.
@Greenduck - you're 172cm with an inseam of 78cm. Your saddle height is 72.5cm?
@Mr A. - you're 176cm with an inseam of 81cm. Your saddle height is 71cmI am 174cm tall, inseam of 81.5cm, saddle height 72cm
@Greenduck - your saddle height seems quite high given your legs are 3 - 3.5cm shorter than mine and Mr. A's. I know these things are all personal and variable
Just making an observation in order to help cdvn make a decision. I just hope you're not hyper extending your legs at the bottom of the pedal stroke. Maybe you dip your foot/ankle more to compensate.
Anyway, in the absence of a bike fit, I would personally advise cdvn to consider a 52 or possibly a 50cm frame, if he can handle the lower head tube. I know what you're thinking - 50cm - short seat tube? No. Supersixes have tall seat tubes: less exposed seatpost. They aren't compact frame geometries. Therefore the seat tube height on a 50cm frame wouldn't look out of place for a 173cm rider.
@cdvn - you have long legs (82cm) for someone of your height (173cm). Therefore your upper body is relatively short. The longer top tube of the 54cm isn't ideal for you. But it would work with a short stem. I say Bike Fit, then 52, 50 or 54... in that order of preference.
Another thing to consider is setback - the horizontal distance between the front/tip of the saddle and the bottom bracket center. My optimum setback, determined after many bike fits and measurements, is around 45-50mm. I therefore prefer frames that give me a seat tube angle (STA) of 74, 74.5 or even 75 degrees. The reason I considered the 50cm SuperSix is its steeper seat tube (suits me) and less reach (again, suits me). But the 115mm head tube wasn't ideal so I decided my ideal would've been 52cm.
Now if I was gonna buy a CAAD8 I would buy a 51 frame. But that's another story