I'm a developer working on the site, and a former MindMeister user, so I can give you my biased perspective. It would be nice if someone who is a user of both systems provides an unbiased one.

advantages: it's free, opensource, doesn't hold you ransom for your data, we aim to create a zero-friction interface

disadvantages: less features. mindmup is a month and a half old project, and we develop the stuff we need + the top voted user demands, so MindMeister has many many more features.