What is the difference between descriptive and analytical writing? Which is better?

In order to break into the top bands in any GCSE literature essay, a student must write analytically, not descriptively. Rather than merely detailing what has happened in a story, one should strive to explain the significance of moments in the story, comment on the effects of individual linguistic choices, and draw conclusions. A simple way to ensure one writes analytically not descriptively is to opt to start sentences with the word 'When' and never 'Then'. 

Example" Fitzgerald's The Great Gatsby

Bad descriptive writing: 

Then Tom arrives at the scene of the accident and, seeing the large crowd, exclaims 'That's good. Wilson'll have a little business at last.' 

Good analytical writing:

When Tom arrives at the scene of the accident his callous, joking exclamation 'That's Good. Wilson'll have a little business at last' demonstrates Fitzgerald's sophisicated use of Nick Carroway as narrator as the line adds a level of dramatic irony. Owing to Fitzgerald's complex narrative stucture, the reader, unlike Tom, is fully aware that Tom's mistress, Myrtle, has just been hideously killed by Gatsby's car. It is ironic that Tom is making fun of the accident because he will be utterly devastated as soon as he realises who it is that died.