Well with Slack, we can integrate it with a ticketing system, so you can assign tasks like group buyers, paying people back, and transportation. In this way you can create the most efficient group outing you can manage. /me shot
In all seriousness, while no one has used Slack, we all have used osu!'s irc system, so thought process wise, it is not a big jump. The only real downside I see is its windows phone support.
On the other hand, about Skype, tbh, I don't use it much. The only reason I have it on my phone is because Byaku convinced last year to use it, and nothing happened. Currently, there is only 1 person I use it to talk to, incredibly rarely, so I'm probably going to uninstall it if we are not using it here. Okay, getting a bit off topic, but I'm saying regardless of which pogram we use, someone will have to use a program unfamiliar to them. I use Hangouts for all my irl communication, but I know that is just me.
Well we still have 4-5 weeks before this convention, so we still have time to test and get people onboard.
Edit: Forgot to say, since so few people actually read the read the thread, it is better to just send out invites for now. It is much easier to talk on those platforms than on the forums. We can have people test how much they like each platform and then consolidate.