I've seen many articles about HOW to install Java. My question is WHICH one should I install for Java development? OpenJDK 6, OpenJDK 7, Oracle Java 6, or Oracle Java 7? Some sites tell me to install Oracle Java 6 AND 7, some say just 7. So, what are the advantages and disadvantages of them both?
It clearly depends on which Java version you might want to develop on. Please see this link for the differences between Java 6 and Java 7. Some sites suggest java 6 because it is considered to be more stable than 7. Please note that anything you can do with java version 6, you can do with 7 too.
If I were in your shoes, I would use the OpenJDK 7, but be sure to download the SDK and not the runtime if you want to be able to do software development with. I do not know if it is 100% compliant with the Oracle version of Java, however it should be enough to cover most developer's needs.