You will find that it is a significantly different kind of fitness involved, particularly in the form of muscular endurancei haven't started rowing yet but I do run everyday. Thats my training regimen for now.
The difference between easy and hard isn't enough to justify this. Why not just do 2km on medium instead of asking a terrible question.
It's not a bad question. Depends on what you are aiming for.
If you want muscle mass and strength then go for short and hard, if you want endurance and long distance training then long and easy (but not that easy!!!).