It used to be that League had professional leagues and Union was amateur, though since Union went professional in 1995. The Union is more widely played around the world while League is mostly popular in Australia and to a lesser extent, England - Union has 15 players a side, League has 13 - In League, play stops and the ball's reset after every tackle and there is no line out for out of bound. - Union has more kicking while the ball is kept on the ground more in League. I'm sure I can think of more...
Normally I would say that the US ought to be able to beat either Russia or Tunisia. Having to put together a side and potentially fly to Siberia, though, would be tough. Just from a logistics standpoint, it's a big task. In that sense the Eagles probably have a better chance of getting through if Tunisia wins. At least our handful of Euro-based players wouldn't have that far to go. I also wonder where the US is planning to play the home leg, as I haven't heard anything. They played a match in Chicago recently, and if that happened again I'd definitely go.