Obviously Dying Curse. Esp. for affliction.
Honestly, take what you need. That much hit on one trinket makes me happy in the pants, but consider this:
The Focus Stone takes away control of your trinket from you. You don't control when it procs.
What if it procs during Heroism or Eradication? Boom, wasted Haste.
For other specs, this isn't so much of a big deal (for example, a Destro warlock with Backlash up will still not beat a 1s GCD on Incinerate) but given that everything that matters to you as Affliction is 1.5s, you will benefit from it less.
But dying curse can proc during lust focus stone will not take effect if lust is active, haste doesn't stack.
A bit of confusion - I'll clarify: Haste stacks, but there is a point where 1 haste = 0 dps.
We all know the GCD caps out at 1.0 seconds. This works out to 50% haste. Therefore, minus heroism, you have a 20% window to play with. Eradication is 20%. So when you have heroism AND eradication, all 1.5s cast spells no longer benefit from haste. For affliction, this means Agony, UA, Life Tap, Nightfall Procs, and to a lesser extent Haunt. Not so much on Shadow Bolt and Drain Soul.
If your trinket goes off during Heroism, you're still waiting on the GCD. Given that Eradication has a ~45 second internal cooldown, theres a high chance that you will proc it during heroism. Thus Heroism + Trinket + Eradication = Wasted haste rating.
The only way this haste isn't wasted is during Drain Soul time, when haste would rock your world. However, my guild blows heroism/bloodlust when its convenient to do so rather than sub-20% to capitalize on execution abilities. If this is your guild, I'd highly recommend the other trinket.
The reason this is not the case for something like Destruction is that they only have 2 spells (worth casting) that are 1.5s base - Immolate and Conflag (Doom has a 1 minute duration so you cast it at most once per Heroism - acceptable but unnecessary to track). Destro locks rarely lifetap too. All of their spells are 2s+, and thus you need closer to 100% haste (unlikely) in order to reach 1s. This way the haste isn't wasted.
TLDR: Haste stacks, but haste trinkets + eradication + heroism/bloodlust = wasted stats.