Between the two I would go with the vial, I think the hit rating would affect your threat more than a little AP. And while not really your question, agris is right, shard of contempt is probably the best threat trinket you can get.
I'd agree, conditionally, with Tattman2an. The real question is what is your hit rating excluding Romulo's? If you are already hitting with all your specials, go for the greater AP boost. But if you are not, go with Romulo.
But his question wasn't which is the best trinkets. He possibly doesn't have either of those two trinkets, and thus is why he's asking between these two. So no, your response doesn't help him at all in the context of this thread. Although it does tell him what to strive for, it doesn't necessarily help him because he might have more important badge picks first, not to mention fighting dps for a shard.
if you have shard, use it all the time anytime you have threat as a component of worry.
Blocker is great for snap aggro, and for a little mitigation but the way expertise works, it's hands down the best threat stat for a warrior. If you can get the shard, use it, but if you're lacking it (like a lot of us are =( ) then use romulo's for threat but make sure you swap it out for when you need survivability (hp trinkets or dodge trinkets).