I'll try simming it both ways, but pre-simming I'd bet money that holding for the CS if it will come off before SD ends is the better option on average, even if it bites you in the ass once in a blue moon.Honestly wouldn't hold it more than a few seconds, just because there is a possibility of it proccing back to back. Wasting procs is like throwing away the winning lottery ticket. I'd say holding it more than 1 melee swing is playing with fire.
Of course using it right away (once you have 50+ rage) will have less RNG swings, which may be preferable for progression.