On this site I decided to focus solely on the Model D RowErg®, because it is the most popular model. The Concept 2 Model E indoor rower ( paid link ) is essentially the same rowing machine as the Model D.
The Model E has a higher seat height – 20 inch (50.8 cm) approx. the same height of a normal chair. The higher frame height means easier on-and-off and it might be helpful if your mobility / flexibility is reduced. It features the same quick-release frame lock connection but has slightly bigger storage dimensions. The monitor arm is fixed and can not pivot. The monitor angle can be adjusted.