The Metrics look is quite pretty and very simple by design. There are few buttons and few options, echoing the design style of companies like 37signals. Much of the complexity is hidden beneath the surface, and the Metrics folks clearly want to build something that "just works." If you POST a new measurement in your JSON package, Metrics lists it with the others. There's no need for updating schema or adding another line to a configuration file -- whew! If the data shows up, Metrics assumes you want to track it.
This approach is ideal if all you want to do is keep track of basic statistics and watch them scroll across your screen. You get two types of data: gauges and counters. The graphs have a few basic date ranges. This confused me at first because I was starting with sample scripts with old dates that weren't appearing. While Metrics is smart enough to recognize new data fields, it loses data with timestamps outside of the current window.
In addition to JSON/Curl and a number of open source collection agents, Librato points to several more libraries and packages for assisting in reporting. The collection is heavily skewed toward modern, trendy stacks such as Node.js, Clojure, and Erlang. But in a tip of the hat to those who graduated more than 10 minutes ago, Python and PHP are also supported. Java? You can be the first to contribute this code, but there's some for handling JMX.
The Silverline half of Librato is aimed at managing servers at the OS level. It burrows into the OS and sets itself up to watch over the applications. While other tools focus on the system as a whole, Silverline wants to drill down into the application.
The simplest thing that Silverline does is report information about resources to their central server. It tracks basic numbers such as CPU load, network bandwidth, and disk bandwidth to help you get a handle on what the machines in your cluster are doing.