Theano vs Tensorflow for building Neural Networks for NLP tasks
At the end this comes down to one's opinion and experience but I ll attempt to give you a decent answer. 

Theano has been around longer and you will be able to find more resources on it. Both to learn and people's published work on it.

On the other hand Tensorflow has been getting better and better since it came out. It has a free online course available and Google even released an open model for natural language understanding.

You may also have to check what hardware you have available. Tensorflow does not run on as many hardware as Theano currently. (Although they are actively working on Windows and OpenCl support.)

A good comparison on various Deep Learning Frameworks can be found there.