For 2018 Cannondale have brought out a couple of Special Edition (or SE) bikes in a couple of their ranges, and here we have the CAADX 105 SE edition.
There is a few notable changes between the regular CAADX 105 and the SE version. Most notably is the color, with the SE sporting an Anthracite finish and tan wall tyres compared to the regular 105 models Silver (shown below).
Along with a color difference there is actually a reason for the price difference. With Cannondales own Si crankset making an appearance meaning a wider gearing ratio with its 48/32 FSA rings compared to the 46/36 setup on the regular 105 CAADX making it a much more suitable road commuter and even better offroad with its slightly more spinny lower gears as both have an 11-speed 11-32 block on the rear.
Another key difference is the WTB Riddler tyres on the SE version, making tubeless instantly an option without having to buy new tyres or rims, as its also specced with WTB's STP i19 TCS 28 hole tubeless ready rims.
So for £100 difference you are getting a significant upgrade in spec overall for not much money. Seems like its definitely worth it.
To see the full differences between the two bikes clock the links below for a full spec sheet.