The explosion of Twitter in 2009 has led to a surge in the use of URL shorteners as well as as a surge in the number of different URL shorteners out there.
There are a few things, from the SEO perspective, you need to know when selecting which URL shortener to use.
First, you want to pick a URL shortener that is going to exist into the future. If shortened links to your site are being posted throughout the web, those are all links that could be in jeopardy in the event that the URL shortener used to shorten them becomes defunct. This happened most recently with tr.im and a lot of controversy followed. Some efforts (most notably 301works) have arisen to remedy this risk, but regardless, I'd suggest picking the most robust URL shortener when shortening your own URLs (if others are shortening your URLs there's little you can do there).
The other thing you need to make sure you do is that you use a shortening service that uses a 301 redirect to your URL (we covered when to use a 301 redirect previously). Search Engine Land had a good post on this topic and a chart that goes over the main shorteners and which ones use proper redirects (as well as other factors you may want to consider).
In short, to make your decision easier, I'd say go with the biggest shortener. They're likely to be using the redirects correctly (or else not so many people would use them) and they're also likely to be a robust solution that will be around into the future.