Be aware that this might be a slightly sensitive topic, but I think it is good that you ask, since many people are probably wondering the same thing.
The firedrake team have made some performance-wise comparison - checkout the video for FEniCS'15. As I remember it, there was not a clear conclusion on that.
Feature-wise, Firedrake is much more application focused that FEniCS and I believe many basic features such as tensor function spaces remain to be implemented.
Christian Jacobs used FEniCS and Firedrake. If I really needed an unbiased and honest opinion, I would ask him privately:
http://christianjacobs.uk/