Publicity hound Joan Stewart points out the following article regarding URL shorteners.
There are more considerations to choosing a URL shortening service than just the ability to quickly and easily shorten your URLs. The ability to shorten a URL has always been important--to the average user who doesn't want to incorrectly copy long and clumsy links, to the affiliate marketer who wants to mask affiliate and tracking links, and now for Twitter users where the 140 character limit requires links be short and sweet.
Danny Sullivan points out that there are a number of criteria you may want to look at.
First is the ability to do a 301 redirect in which the server tells the browser that the short url has "permanently" moved to the long URL. In a 301 redirect, the search engines give link credit to the short URL.
Some services offer tracking. You can get tracking stats whose depth varies by service. This is either a nice plus or critical information.
Then, you want to see link stability. A broken link is useless, but gauging link stability is difficult. You are dependent on the stability of the service--if they cease operations, say good-bye to your links.
Twitter client support for those who use Twitter clients such as Tweetdeck.
If using the service for Twitter links is important, then you want a service with the shortest possible domain name to use the least amount of characters. Tinyurl, for example comes in at 19 characters versus bit.ly which comes in at 13.
The complete article which compares the various URL shortening services can be found here:
http://searchengineland.com/analysis-which-url-shortening-service-should-you-use-17204
