NetLogo look very good for beginners, but this is "dead end".
It have unique, strange language and biased philosophy of model design. From any JAVA based environment you can switch quite easily to any other based on JAVA, C# or even C++ or CUDA (when you need higher efficiency and scalability), from NetLogo you have to learn almost from scratch.
I do not have any experience neither on GAMA or Janus, but you might want to take a look at Presage2 (http://www.presage2.info). It allows to do more complex things than the usuals ABS platforms such as Repast or NetLogo. It's written in Java, and while the learning curve might be a bit steeper, it's much more powerful.
I am not sure why you have not looked at JADE (JAVA Agent Development Platform). This is one of the most widely used MAS platform with a active community support.