Diction is the choice of words based upon the style of one's writing. Tone, on the other hand, shows a writer's attitude toward a specific subject.
In regards to Ellen Goodman's choice of diction (to create the tone of the article "In Praise of a Snail's Pace"), her diction mirrors the subject of her article--slow, like a snail.
Goodman's word choices illuminate the fact that some things are better done when they are done slowly. Given the pace of the world today, influenced by technology, Goodman makes the claim that, sometimes, the world needs to be slowed down.
In regards to specific things, like thank-you notes, wedding invitations, and notes of condolence, the fast-paced world of technology is not appropriate.
Throughout the article, Goodman uses words such as "slowness" (in opposition against "whirlwind") to depict the importance of slowing down in the world, especially when it comes to things of a serious nature. In an age pushed ahead, faster and faster, by a sense of urgency, Goodman's article shows the importance of "stopping to smell the roses," to use an old adage.
Therefore, her choice of diction ("fast" words contrasted against "slow" words) proves to be perfect.