Which one should be used for web application forms? As a rule, the HTTP protocol defines POST for things that may have side effects, and GET is for read only viewing.
tls and sniffing ninjas
If an eavesdropper is able to sniff your traffic it does not make any difference if you use GET or POST. The POST and GET parameters are both in plaintext inside the HTTP message.
GET parameters are cached in the browser history.
A web server typically logs GET parameters but not POST parameters. Server logs may be stolen or archived.
Browsers typically warn the user if hitting the back button on a POST submissions. This is to prevent duplicate form submissions. This is a good thing.
When GET requests have side effects, a URL can be put inside img html tags. An attacker may put this in his profile picture:
Yes, any html page can do this, but using GET makes it a tiny bit esier to cause havoc.
HTTP referrer leakage
If the resulting page after a form submission contain references to other servers’ resources, the GET parameters are getting leaked. E.g. google analytics and others.