The KT34M2 is bigger, heavier and a better performer on 10-15-20m. The C3SS is smaller, lighter and much easier to assemble. I've had and used both.
Where we are in the Cycle today, I probably wouldn't put up either of these, as I'd want something with gain on 17 meters. 17's second only to 20m in propagation and these days it's all too common that 10-12-15 are just "closed." I'd probably rather have a 20m/17m antenna than one for 20-15-10. Of course, I'd change my mind about this, five years from now.
WB2WIK/6