Late to the thread but......
There was a period earlier this year that I was putting @16k/week on a C2. They were split on a D and E.
E pro's - More solid for the rower 90kg or more in BW, taller for getting in and out of in intervals.
E Cons - $$$, takes up a bit more room
I don't think there is enough of a diff to justify the cost of a couple sets of bumperplates.
If you plan on doing indoor rowing races or training clients w/ the occasional race, the bluetooth on the PM4 monitor might be worth $200.