Gnomish in my opinion, it gives better options and also the Gnomish Lightning generator is really good for some classes, it can also be reforged.
To clear up some confusion the Gnomish lg isnt from gnomish engi....any engi can make it...the diff b/w gnomish and goblin is that gnomish is fun toys and goblin is fireworks basically...endgame (twinks for that matter) it doesnt make a diff
Almost everything is BoE, so you can just get goblin/gnome engineers to make stuff for you.
You just need to decide what stuff you'd rather make yourself and look at the BoP items.
Again the only item that is actually useful in arena (that is exclusive to gnomish or goblin engineering) is Goblin Rocket Launcher.
And while it is BoE, it also requires you to be goblin engineer to actually use it. So once again: goblin is the way to go if you're planning on using the rocket launcher. If you're not planning on using it, it doesn't matter what you choose.