There's been some comparisons in the past.

http://logz.io/blog/fluentd-logstash/

https://www.pandastrike.com/posts/20150807-fluentd-vs-logsta...

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

They are all much of a muchness....fwiw distros that ship with rsyslogd as the syslog daemon are in a good spot, since rsyslog can do sufficient routing and spooling to account for network interruption. One less thing to install. And it supports the latest structured syslog RFC.

That said, I use nxlog basically everywhere because I run a heterogeneous environment and it works well on all OS's I use (and is fast and light on resources)