To me, stability is the biggest selling point of Goo.gl over alternatives - it's run by a company that isn't likely to disappear next year and is known for having some of the most scalable systems in the world. I know the team has worked hard on making this as scalable and reliable as most anything at Google, including search. I think it's safe to say Goo.gl links are any one's best bet as far as future-proofing your shortened URLs. Also, because of Google's obsession with end-user speed, this is likely the fastest URL shortener you are going to ever find. You don't have to take my word from it, see this post from TechCrunch a few months ago that shows Goo.gl as the fastest and most reliable from two different third-party analyses.
|Reliability, Greener is Better|
|Speed, Smaller is Better|
Of course, I'm the first one to admit that I'm disappointed that we even need URL Shorteners, but if you assume that you indeed do, this is a great choice.