not to say—is used to imply that one is content with a more moderate statement than that which might have been made:
- It is one of the handsomest counties … the farmers are as a whole well-to-do, not to say rich.
to say nothing of—is used to emphasize the point by adding extra information:
- Much damage is caused to table tops by spilled cosmetics, to say nothing of marks caused by that earlymorning cup of tea.