The best cross section of a turnip really depends on personal preference and the intended use of the turnip. For example, if you're planning to roast or sauté the turnip, then a thicker cross section may be best to help it retain its shape and texture. If you're using the turnip in a salad or soup, then a thinner cross section may be preferable for ease of eating and mixing with other ingredients.
Ultimately, the best cross section of a turnip is one that meets your specific needs and preferences for the particular dish you're preparing.