My recommendation is that you use a live text chat whatever else you use. Generally speaking most video chat clients also include a text chat.�� A live text chat would allow for more people to participate especially people on slow connections. Of course the more people join on a video chat the bigger the latency. Voice chat is also tricky for long conversations because you have no indication when someone wants to talk or when someone is about to talk so people tend to talk one over the other which happens also on video chat too.
So by starting with a live text you allow a lot of people to participate to the coversation, also in a live text chat you can have several conversations going on at the same time which makes things also a lot easier and faster.
I have done Google Hangouts in the past, it was ok, the advantage of the hangouts and video chat is that it is dead easy to share a screen so its easy to show people what you talking about. I would not try Google Hangouts or any other video chat with more than 10 people, it would be crazy 20 people trying to find an opportunity to speak and the latency making things harder.