Wow, this is kind of a loaded question.
What type of flying are you wanting to do? Something more precise, or something that's quick and flies like a NERF football with wings?
The Dart is going to have more precise, 4 channel flight due to the potential of differential thrust, and will allow you to do some fun flat spins. I've built the Sea Duck, and love it to death because of how it snaps around with that diff thrust from the twin engines. It's also got a little bit more power, so if I'm flying in wind, I can bring it around a little easier. That Dart's twin motors will have similar characteristics.
The Goblin, however, is going to fly faster. Its shape will allow you to do a lot of bank n' yank, "crank it over and pull back" type of maneuvers with it. It's going to fly more like the Versa Wings, and has the capability to be stupid fast. Keep in mind, though, that "stupid fast" means it requires multiple rates to keep it under control. The faster you move through the air, the less movement it takes to make it change direction. The steeper an angle on the aileron/elevon, the faster and harder it's going to pull around, and the more squirrelly that thing is going to feel. I speak from experience with this one on my Nano Goblin - that sucker's FAST, and can fly for 30 minutes if set up correctly. The larger FT Goblin probably won't have those same flight times due to the less aerodynamic shape and size, but you can probably expect decent times out of it...
Ultimately, kinda depends on how you want to fly. The Goblin's that NERF football with wings, the Dart's more of a Frisbee, if that makes sense. I don't know how better to describe it.