Javadoc: package.html or package-info.java
Solution 1:
package-info.java: "This file is new in JDK 5.0, and is preferred over package.html."—javadoc - The Java API Documentation Generator
Addendum: The big difference seems to be package annotations. There's a little more in the way of rationale in 7.4 Package Declarations.
Addendum: The annotation feature is also mentioned here and here.
Addendum: See also What’s package-info.java for?.