It is up to you and your teaching aims which one to choose.
However, the plethora of in-world affordances and online educational tools available can make it an unprecedented experience for both your learners and yourself.
Here is a machinima that outlines how you can employ virtual worlds in class.
You can go exclusively in-world and have real-life (or unreal) simulations with your avatars. (Adult beginners)
You can have grammar lessons and promote language in use. (pre Intermediate)
You can recycle language or literary material through immersive language games.
You can help learners practise language simply through the use of the chat box.
You can teach shapes, colours, textures through building in-world. You can teach them to add script, texture, animations use HUDs, teleporters, rezzers.
Learners can make their own machiniLOG about their everyday learning experiences.
There is an inexhaustive list of activities you can do with your learners.
This is the first in a series of posts on how to teach in a 3D Virtual World.