CUSHION CUT
Cushion cut diamonds have rounded corners with larger facets to increase the brilliance of the stone. The larger facets will highlight the clarity of the diamond. I would recommend going for a higher clarity in this type of diamond due to this reason. Cushion cut diamonds are available in both square and rectangle. The length-to-width ratio will effect the look of the stone from above. This Ratio is shown in the diagram. 
For a square Cushion cut diamond the Length-to-width ratio should be between 1- 1.05. However if you would like a more rectangular shape for your design look for a length-to-width ratio of more than 1.15