The above is a fairly lengthy explanation.
The tl;dr is the MyItem.sendCommand and postUpdate methods can be smart and handle different types of states more intelligently than the sendCommand and postUpdate actions. So unless you can’t for some reason, always use the method.
Sadly, as watou and I were discussing here there isn’t a good set of documentation for the Rules DSL language. However, this particular issue isn’t a language issue but an openHAB issue.
The best I can offer are:
http://docs.openhab.org/configuration/rules-dsl.html
And look at examples.