bianuzzi22 - Identical concept and specified performance.
The main difference is the midrange driver. The original CS3 Vifa was upgraded to a custom ScanSpeak midrange that allowed broader (7 octave) coverage. Also, the CS3 woofer got a polymer coating which smoothed out its upper range. Crossover tweaks were necessary and the baffle angle is a little different. Same tweeter. The 3.5 EQ is a little more sophisticated. The CS3 sold 2000 pair and the 3.5 sold 5500 pair. I consider the CS3.5 as quintessential Jim Thiel. I am bundling the CS3 and 3.5 together. All upgrades will apply to both platforms.