What makes one short URL better than another

If the goal of URL shorteners is to make a URL as short as possible, that would mean that is.gd is whopping 60% better than TinyURL.com.

Personally, I use Bit.ly because of the neato bookmarklet (it is also has a short name), but other shorteners offer different features like previewing, email cloaking, etc. You can check out over 90 different services here.