Okay, I'll wait some time if some of the other players have anything against Obsidian before setting up the campaign there properly.
Also, about the rule system - I'd prefer the Cortex System. It's not free, but since it's not available in German, there is a German summary of it found
here (Serenity RPG, deutsche Kurzregeln und Tabellen 0.93). If you don't want to download the whole rulebook "somewhere", I'd advise you to try to read that with the help of your translator.
I used it before and I like it because it's a bit more light, free and fast than other systems I know.