If you like rolling around on the floor getting on and off a rower then get the model D not to mention kicking up the dust off the floor.LOL I really like the higher model E in that every 5000 or so meters I will take a water break and just relax while pushing the seat back forwards and backwards with feet on the ground. Good luck and happy rowing with either model.
I just completed the Indoor Rowing Instructor certification course (Concept2 & UCanRow2) and had the opportunity to evaluate all of the current models. I am an older age-group athlete and I think the Model E is perfect for older athletes or if you already have mobility issues. Chair height seat is the way to go - if you want to have confidence in entering and exiting the erg. Well worth the $200 bump in price - same high-quality experience.
It is set higher off the ground. It makes it easier to get on and off. Also, some of the components are made for gym use. A little more heavy duty. I have had mine for about two months and am glad I spent the extra just for ease of getting on and off.
The Model D is much easier to live with, cheaper, comes in a nice black colour option, and angle of the display is adjustable (the display arm pivots). On the Model E the display arm is fixed, so if you don't like the angle, there is nothing you can do. The Model E is really only worth paying for if you want a higher seat position, which you only need if you are very large (say 400+ pounds) or have mobility problems (you can't sit down on the floor or get back up again easily). Update (September 2015): model E is now available in black also (see Concept 2 web site).
Not much of a difference overall, except the Concept E is 20 inches off the floor, whereas the D is 11 inches, I believe. It is much more comfortable getting on/off the rower at the higher height...it has been worth the extra $200 to me... The E also only comes in the grey color, whereas the D comes in grey or black...hope this helps...
The model E is a taller machine. Chair height which helps me since I have a full knee replacement. I saw the assembly instructions for the D model and the model E seems to be much sturdier. I'm over 250# is this machine is definitely sturdy.