It would be the jewel, albeit I don't have it. Besides the set bonus it's a place to slot those nice talismans anyone with sentinel uses with main spec gear.
With 3pc warlord you trade 104 magic power and 130 off stat debuff/130 init buff proc for 6% magic crit.
I compared all possible combinations with focus on intel, magic power, wounds and magic crit and came to that conclusion. Even without 4pc ranked I wouldn't pick the added crit, as crit rates are already pretty high and all your crits will hit harder with added mp, while intel is capped at 1050 anyway.