These three services have different strengths, so which you choose strongly depends on your needs.
For example, the video conferencing aspects of Glip were less than stellar (and non-existent in Flow, which has a different focus). Slingshot is useful for video conferencing and screen sharing, but it needs more maturity before it can compete with other low-end but free video conferencing tools such as Vyew or Join.me, which both support video conferencing, screen sharing, document sharing and threaded discussion spaces for up to 10 users.
On the other hand, Flow handles task management quite well, along with some rudimentary file sharing.
In the end, however, Glip is clearly the most capable of the three and could probably make your team more productive and cut down on lots of emails about scheduling, task management and sharing document versions. In fact, Glip stands up successfully to some of the heavier and older collaboration tools such as WebEx and GoToMeeting (provided you keep the group size small). It is particularly useful for groups working on visual projects, such as art directors and photo editors.
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