Bugfender is mainly focused on getting the application logs, which is very useful for debugging and reproducing errors reported by users. Also has a "user feedback" option, that lets you get feedback from your users and get the application logs alongside it. Bugsnag and Crashlytics are more focused on "error analytics", mostly crashes, and they're more advanced in terms of analytics and detection of trends in crashes. I hope I helped! Disclaimer: I'm one of the founders of Bugfender.

Bugsnag is one of my favorite tools. I must admit that I can't compare with the other two variants, because I only have experience with Sentry and Bugsnag. Both are fine, but I lean towards Bugsnag. With the help of Bugsnag I'm able to see quite clearly what went wrong, and it shows the affected users (and in my case even the tenant as we're operating a multi-tenant application).

Regarding Logstash: I've been using Bugsnag as a complementary tool with Logging + APM platforms like NewRelic or Datadog. What I mean is, go for Logstash if that's the log monitoring tool you like, but Bugsnag and Logstash are not interchangeable, and it perfectly makes sense to use both.