Duststorm used to be every tic 15 sec before LU 13, Never understood why it was changed to such fast tics, with situations where its useful now are so rare, only spot i really found it useful was way back in DoF on the last floor of PPR on the large orange skeleton groups.
As for Verdant Sigh (42) Vs Verdant Grasp (70) There is zero reason to waste the extra power curing if you have M1 cure of Verdant Sigh, as it will cure any mob level 76 or lower. However, if you have the Remedie Line AA, the higher version you use, the better bonus you get
Verdant Sigh (42) (228 Power) *Note i have a bit of Ministration so the power cost may be higher for you.
315 Physical Mitigation
630 Elemental
Verdant Gasp (369 Power)
525 Physical Mitigation
1050 Elemental
If you dont plan on getting the Remedies line, there is zero need to waste more power, unless you encounter a mob Level 77 or higher. (I dont have the 56 version as never saw a need to buy it, its obviously somewhere imbetween the two)