Amon appears to have a slightly different goal set than Monit. Monit is more of a "Is this process running and ok?" tool for keeping processes running as expected. Amon seems a bit more like Munin.
Monit is a bit more than that. You can also use Monit to check remote hosts. Not only can it check http, but ftp, ssh smtp and more. You can use Monit to test your web-application via the HTTP protocol (404 or 50x) and use regex to check if certain content exist (or not) on a web-page. Monit is free, open source and does alerts for free.
Thanks for the clarification, but my intent wasn't to describe the full feature set of Monit. All those things you described are still limited to the scope of "is X running?" type questions. Can Monit graph time series data?
EDIT: My point isn't that Amon is "better" than Monit, only that it's different. There is some overlap in feature set, but ultimately, the tools have different goals.