I think AndyF has, maybe not I've dyno tested motors with both but not on the same motor I think the 3X is for large C.I. motors, probally above 511 C.I.
the one motor I assembled and dyno tested the 3X was a 4.5x4.5 World block with right at 15.0 to 1 compression on race gas, it had uneven fuel distribution problems on # 2 and #7 cylinders according to the EGT and spark plugs Bob Goerge could, and can, probally fix that