GET is a lot faster, but often considered less safe than POST.
The reson is that GET is part of the URL, so it shows up in logs from your windows box to the coffershop's router to your ISP's logs, to the site's logs...
POST is not logged as such, and handles more data than GET.
Still, GET is easier to debug because the url (and all those logs) contain the actual data you passed.
You can send GET from any domain to any domain without security warnings, whist POST usually requires a real <form> and security click-through to do xdomain transfers.
if you use ajax, neither can talk to other domains.
using an image ping, you can GET to anywhere, but not from anywhere...
so, it depends on:
1. how much data you have (+2kb needs post)
2. where it's going? (same site or elsewhere)
3. security implications (don't login to a bank at starbucks using GET to submit)
(2013/5) HTML5:90.2% MOB:14% IE7:0.5% IE8:8.6% IE9:9.8% IE10:10%