I use both Darts and Firebombs, so for me it's really Firebombs vs. any other AoE spell the WD has.
I just don't AoE everything down anymore, it's not effective enough. Darts, Haunt, and Spirit Barrage make up my offensive capabilities and it works wonders right now in Act 2 Inferno.
Honestly, the strategy changes A LOT for Witch Doctors come Inferno... at least in my case it did. For most of the game pre-Inferno Witch Doctor was about Soul Harvesting everything imaginable and then using Dire Bats, Firebombs, Zombie Dogs, Zombie Bears, Acid Rain, Raid of Frogs, GotD with Rain of Corpses, and whatever else I could AoE with. I really didn't single target anything but there was a lot of area damage, a lot of getting in the fray, getting hit a few times, and then siphoning life and retreating with Spirit Walk. My Gargantuan was also a huge part of play, especially for Champions and bosses.
In Inferno I use Darts and Spirit Barrage which massacres practically every normal mob instantly - taking down an entire group is pretty simple but it's all just shooting each one individually really quick. When it comes to Champion packs it's ALL kiting ALL the time, in that case Haunt is MAJOR in dealing damage over time while I run around with them, I throw out Zombie Walls whenever I can to hold them in place and whittle them down with Darts/SB and save Spirit Walk for when I need to deal with walls/frozen/jailers. I use Horrify to fear whenever possible as well, let them run for a few seconds and then blast them to hell until they come back and I have to continue kiting.