Sentry

I think Sentry is far more user-friendly and has a better UI. Sentry is also a more affordable option.

Sentry was cheaper and lighter weight/easier to deal with. New Relic always felt like it was slowing the site down some. I don’t think either has had any major negative impact, but Sentry always seemed better/faster. Also, Sentry doesn’t have contracts like New Relic does (typically 1 year).

Sentry is better suited for tracking and aggregating exceptions over New Relic. New Relic does report on exceptions that occur, but Sentry is better at rolling up similar exceptions and filtering out the noise. Sentry also does a great job at identifying when an exception first started occurring and sorting by recent/most frequent exceptions.

Sentry is really a tool to be used in combination with other things, like Pingdom and PagerDuty. For those applications, Sentry is a far more full-features offering that lets you see why errors happened, not just be alerted to their occurrence. We chose it over other error monitoring applications based on flexibility and ease of use.

There are quite a few players in this space, but Rollbar and Sentry seem to be the top two. I can't remember why I chose Sentry over Rollbar, but they seem pretty close in terms of features.