You can't go wrong with shards at 20. In terms of gaining and holding aggro, warriors need to rely on proccing weapons far more heavily than other tanks who have the ability to gain aggro via spells (like Disease Cloud
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Additionally, Obsidian Shatter was recently given a nice aggro buff (along with Ykesha and a handful of other procs). To the best of my knowledge, Obsidian Shatter is a 30 point dd with an AC debuff, certainly nothing to scoff at when you're level 20.
Edit: To answer your proc-vs-damage aggro question - it's all about the procs. You'll never outdamage a monk, and my higher damage weapon set of Jarsath Trident & Cane of the Tranquil did nothing to hold aggro with monks or rogues in my group.