The first syntax defines a task, and provides some code to be executed when the task executes. The second syntax defines a task, and provides some code to be executed straight away to configure the task. For example:
task build << { println 'this executes when build task is executed' }
task build { println 'this executes when the build script is executed' }
In fact, the first syntax is equivalent to:
task build { doLast { println 'this executes when build task is executed' } }
So, in your example above, for syntax A the description does not show up in gradle -t because the code which sets the description is not executed until the task executed, which does not happen when you run gradle -t.
For syntax B the code that does the ant.echo() is run for every invocation of gradle, including gradle -t
To provide both an action to execute and a description for the task you can do either of:
task build(description: 'some description') << { some code }
task build { description = 'some description'; doLast { some code } }