My logic to which trinket it beter.
[Mjolnir Runestone]
Equip: Impoves critical strike rating by 102
Equip: Your melee and ranged attacks have a chance to increase your armor penetration rating by 665 for 10 sec. (The Proc Chance is 15%)
[Needle-Encrusted Scorpion]
Equip: Improves critical strike rating by 114
Equip: Chance on melee or range critical strike to increase your armor penetration rating by 678 for 10 sec. (The Proc Chance is 10%)
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Both have 45 Global cooldown. My assumption is based on the Proc chance and what procs on I see Runestone with a higher uptime = Beter