The aug adds 600 damage to the next spell you cast. It also returns IIRC 250 mana back? No other augment can do that. 30 SD is less than 600 damage, and 30 hINT is less than 250 mana back.
On our standard rotation (RS -> Of Many -> Chaotic -> Spear), the Prayer Shawl has a ~23.4% chance to proc and add a 600 point SPA 286 (post-crit) modifier to our next spell. That's only a 400 point increase over Illusion Benefit Greater Jann (which has a 200 point SPA 286 modifier), and it does nothing for the duration of Spire of the Minstrels. It would also never focus our fire spells if you had a Bard playing Constance's Aria, but I'll ignore that given doing so is not optimal. If you assume a 30% effective Twincast rate, on average you'd gain 115 damage on the next spell, or 92 damage on average accounting for Spire. On the same weave with the same Twincast rate, just on our primary spells (excl. procs) and assuming just our passive crit rate/chance, 30 spell damage would add 155 average damage per cast. In a raid scenario on Kizant's webapp (still ignoring melee procs) with all the normal casting procs and Auspice + FE, 30 SD would add 676 damage per cast. On a full burn, it would add 1,280 damage per cast. The mana return is definitely unmatched by any stat aug, so it really comes down to whether you need the mana or not. You aren't using the Shawl for its damage bonus.
