Both weapons are great. If cost is no object, then I prefer Last Wish. If cost is a concern, then you really don't get enough benefit from Last Wish to make it worth the runes.
The nice thing about Last Wish is that, yes, it's true you can get all of its benefits from other equipment, but then you're locked into using those specific other equipment options. Last Wish gives you choices.
For example, you can get Fade from Treachery. OK, but now you're stuck using Treachery for your armor (or even worse carrying a second armor and wasting the time to swap armors in and out). With Last Wish, you can use Chains of Honor, Fortitude, or whatever other armor suits your fancy (Duress?). You can get Life Tap from Dracul's Grasp and/or Exile, but now you have no choice on what gloves or shield to use. If you use Last Wish, you can swap in a different pair of gloves (Blood crafted with IAS) or a different shield (Herald of Zakarum). You can get crushing blow from Guillaume's Face, but you already told us you have some problem with that item. What if you want to use Kira's Guardian for resistances or a Crown of Ages for damage reduction or any number of other helmets?
If you're happy with the exact set of equipment that you'll need to make Grief work, then there's no question that it's better. You'll do more damage. But if you prefer to try some other ideas, then Last Wish will let you do that much more easily than Grief.
Furthermore, don't forget that somewhere around 90-99% of the damage you do is going to come from crushing blow. Yes, Grief will let you do more damage with Smite, but so what? You'll probably kill the ubers 3 to 5 seconds faster, if that. So, again, it's true that Grief is faster, but if you like the idea of what Last Wish brings to the table, don't let someone convince you that Last Wish sucks because of all the damage you're "sacrificing."
Edit: In the end, any weapon with crushing blow is a good PvM smiter weapon. A Black rune word flail is actually surprisingly good, and for the price you can't beat it.