Yes, both matches are pretty hard to predict. That's why I will not bet anymore on the World Cup games. It was much easier in the group stage event though there were some surprises.
Predicting the Argentina vs Netherlands match is much more difficult than Brazil vs Croatia. The Netherlands has the advantage of being expected to win against Argentina even though the probability of Argentina winning is still higher. However I can expect Argentina to win that match over the Netherlands and that is my initial prediction for Argentina. Currently Argentina has @1.3 odds, while Netherlands @2.5. For fun during the match, I think it's safe to bet on the victory of one of them.
While things are clearly different for Croatia, they really weren't expected to win against Brazil. I have looked at several betting books and all are favoring Brazil to win so the odds for Croatia are really high. On some sites, the Croatian odds are close to @8, that's clearly a trap for bet.