Yes, there are distinct differences between KT66 and 6L6GC. The 6L6 are warmer and softer - the KT66 more detailed and dynamic. Which is better depends on the rest of your system and personal tastes.
There are really only two versions of the tubes anymore: The Russian/Slovak (New Sensor) and the Chinese (Shuguang). The Russians are darker and heavier sounding compared to the Chinese in general. I prefer the Chinese. The Valve Art and Groove Tubes are screened tubes for lower noise but are all made by the same Russian or Chinese companies.
My favorite so far in my MC240 are the (Shuguang) Groove Tubes KT66C. They have more transparency than the (New Sensor) Tung Sol KT66 and have the benefit of being a true kinkless design. The (New Sensor) SED and EH 6L6GC are quite warm and soft IMO. The (New Sensor) JJs are more microphonic than the others in my experience.
Be careful that if your amp is only designed for 6L6GC, you CANNOT use any of the other 6L6 tubes because of excessive anode voltages. Many of the NOS 6L6 are not GC variants.