In short union has contested breakdown (if a player is tackled, player must drop the ball and both sides go for it) whereas league doesn't. League stops play after a tackle, has 13 a side instead of 15, 6 in scrum instead of 8. League scrums aren't contested so it always goes to the side its awarded to.
League is more fun to watch, union is more fun to play.
Union and league are pretty different, but the main difference is that with league, you have only 5 plays before you got to kick the ball. So it's very fast paced as you don't have time to keep possession like in Union. Anyway, my money is on all blacks but if they choke (which is possible), I would go Ireland. I think Australia may cause an upset too.