If you're worried about the shortener not disappearing, I would recommend sticking with Bit.ly. Betaworks isn’t about to pull the plug on a service that integrates so nicely with another one of their projects (Twitter). Also, they have plans for the future (like the "Bit.ly Now" system which TechCrunch calls a Digg killer.)

Ow.ly isn't likely to go away anytime soon either. They seem committed to what they do.