EtG is the most common. After that, it's oEtG since it's more fleshed out than Cygnia is. Cygnia is very specialized.
You can start with any of the three, although yes, I would say that learning the basics of EtG would be a good way to start.
And there isn't really any risk AFAIK. Zanz does seem to still support the game (and he even came back, albeit for a moment, to fix the game when a database crash occurred earlier this year), so I think EtG will be here to stay.