[kuchel]

And also don't forget those situations where more consistent damage kills better.
For example, the situations where, you aren't strong enough to kill a monster in normal 3 hits and require a 4th hit - calculated strike - to finish it.
With Onslaught, 3 hits all with 50% or more damage would be able to kill a monster in such a situation. So sometimes spreading out your damage bonuses between every hit is better.
It depends a lot of their HP and your damage (after taking into account resistances). So you can see that neither of the skills surpasses the other by much in this respect.
But then... Onslaught has all those extra bonuses (Ardor and Ignore Pain). And Calc. Strike only has measely Lucky Hit, which I showed doesn't become useful all that often.
And like I said, Onslaught damage bonus applies to every other phyiscal attack you do thereafter (Lethal Strike, Warwind) and Calc. Strike doesn't.

The only thing I think Calc. Strike has over Onslaught is 0 mana cost. But hey, 1 mana is really nothing.