(Disclaimer: I am a Fluentd maintainer)

Here are some comparisons out in the wild (Note: neither of them is a Logstash or Fluentd maintainer afaik)

- http://jasonwilder.com/blog/2013/11/19/fluentd-vs-logstash/

- https://blog.deimos.fr/2014/05/13/logstash-vs-fluentd/

Also, if you have any question or doubt about Fluentd, please feel free to email me at kiyoto@treasure-data.com