Joined: Jun 2019
Posts: 1,626
Likes: 1,787
From: Near the Heart of OH
Bikes: '25 Jamis Renegade S1, '18 Quick 1,'04 Trek 2300, '97 730 Multitrack, '95 750 Multitrack, and a few others
The Synapse SE was a one year only model and I regret not purchasing one. Wider tires (700Cx35) and 11/34 cassette along with some other things set it apart from the regular alloy disc 105.
As to size - I’m 5’6” with a 29.5” inseam. I tried a 54 but standover was really uncomfortable and reach was too much. I have it since tried several 51cm Synapses and ended up purchasing a 2016 105 Alloy frame and built it as I wanted. I think a 54 would be too small for you…
I can also speak to the CAADX - it feels much larger than the marked size and I passed on a friend’s CAADX for that reason.