Ranged Crits are just a 25% damage multiplier (without Dead Aim, which doesn't apply to WSs), while other melee jobs get something more like a 2x damage multiplier from crits against high defense/level enemies. The defense of the enemy has no bearing at all on the value of Ranged Crits.
AGI has been shown to increase ranged critical hit rate (which is why Jishnu's should have been an AGI mod). However, the amounts of AGI that you need to do this are astronomical. You're probably going to pull a 10-12% base ranged crit rate against most high level stuff you fight. Jishnu's innate crit rate boost has not been quantified, though it should be pretty easy to do.
Using these sets:
127 Base damage (Adaman Bullet) + 76 Base Damage (Annihilator Level 99) = 203 base damage before WSC
Last Stand :
http://www.ffxiah.com/item-sets/283385 (+46 STR, +94 AGI, +62 RAtk)
- 110 VIT enemy on RNG/WAR with an RCB and assuming perfect TPing and augments: 1711 Base Damage*fTP
51 Base damage (Gargouille Arrow) + 117 Base Damage (Gandiva Level 99) = 168 base damage before WSC
Jishnu's Radiance :
http://www.ffxiah.com/item-sets/283384 (+77 DEX, +67 STR, +36 RAtk)
- 110 VIT enemy on RNG/WAR with an RCB and assuming a 10% base crit rate boost from Jishnu's and 5% from Fencer with perfect augments: 1822 Base Damage*fTP with crits
Things unaccounted for:
* Annihilator has more RAtk, RAcc, and access to a useful aftermath. If the Velocity Shot enhancements work on WS (unknown then it also has that)
* Jishnu's base crit rate is unknown. I assumed 10%.
* My sets are very likely non-ideal. The question is whether they're more non-ideal for one WS than the other.
Edit: For the record, Rancorous Mantle is a minor improvement over Vigilance Mantle +1, if it is even better at all.