I have both. I like them both a lot. It's hard to say which is better -- I've run both on the street and the track and both are plenty stiff.
I ran the Sugino 75's followed by the Campy's later (the new ones, I also have the old ones and those are nice too) and felt a difference in stiffness, with the Campy's feeling slightly more stiffer than the 75's.
I attribute this to "feel", much like how a frame will feel and flex differently than another, the cranks have a feel to them too, aside from stiffness. The Campy's feel lighter in some sense, perhaps due to the larger and bulging spider, and I feel as if the q-factor is slightly bigger (111mm BB spindle versus 109mm).
Then there are of course aesthetics -- do you prefer the understated workingman's non-nonsense minimal appeal of the 75's or the flashy and shiny smooth curves of the Campy?
I do feel, at the older price, that the 75's were good bang for the buck. Nowadays, the difference isn't much but if you wanted to go for visual appeal, I'd go Campy.
I do feel that perhaps the Campy has a slight advantage overall. If I had to choose based on experience, I'd go with the Campy.
I don't think you can go wrong with either though.
Edit: Caveat emptor and to each their own. I like all of my cranks.