I recommend you to go with Castalia. Have a look. You should enjoy it. But, remember to work with Castalia you should have good understanding of C++ programming. BTW, Castalia is built on top of OMNET++.
You can also give an overview exactly what you want to do? Then I could probably suggest you based on my experience whether using Castalia would be the best option or not.
Exactly as Ahmed is saying you should first decide that what you exactly wish to do in WSN. In my experience Tiny OS is also nice simulator for WSN if you are dealing with protocols.
If you are dealing with delay tolerant network in WSN. Then One simulator is the very easy and best solution