Choosing between TensorFlow or Theano as backend for Keras
It was announced on  2017-09-28 that Theano will be discontinued:
From here (Yoshua Bengio):

After almost ten years of development, we have the regret to announce that we will put an end to our Theano development after the 1.0 release,  which is due in the next few weeks. We will continue minimal maintenance  to keep it working for one year, but we will stop actively implementing  new features. Theano will continue to be available afterwards, as per  our engagement towards open source software, but MILA does not commit to  spend time on maintenance or support after that time frame.

So TensorFlow is a better option.