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Goblin seems to be the favorite from people that I know.
I have found the exact opposite. I know of almost no one who takes Goblin engineering. The reasoning is because the explosives made are just too darn expensive; whereas the items made by gnomish are usually only limited by cooldowns, not by materials. Finally, when Gnomish goes wrong, funny stuff happens: you get an effect on you when it backfires (ie: you get netted to the ground instead of the enemy, or you get Mind-controlled). When Goblin goes wrong, you set yourself on fire or blow yourself up.
For the BoPs:
Dragon Gun versus Death Ray: The Dragon Gun does less damage to a single target, but can do decent damage to multiple targets (overall). It can explode, and you get set on fire (fear effect and DoT). The Death Ray drains life to do damage to one target. At level 30, my highest hit was around 2000 damage (critical hit). If you get stunned, or move, you've still taken the damage but the ray doesn't activate. I still prefer the Death Ray: it does major damage, has a decent range, and is perfect for anyone not taking direct hits (like my Warlock).
Battle Chicken versus Bomb Dispenser: A fighting chicken just suits me better. The Bomb dispenser is basically a free high-power explosive sheep every 30 minutes. The battle chicken is an NPC pet.
As I said before, even if I was a Goblin engineer, the material cost of the bombs can get too expensive to be worth making later on. I like Gnomish ^_^