Man, I cannot believe I missed this thread!!! Rowing on an erg is NOTHING compared to rowing on water. Firstly, the scenery is amazing, you get to see fish below you, the water can be as clear as a crystal and the sunrise is phenomenal in the mornings. Rowing is the best, hands down!!! Secondly, you get to row in a crew, where you get to form close relationships with each other, and you also get to participate in actual races.
The only downside to that is that you have to wake up early to get to the river at 5 (for me, that is - you don't have to do that!), but I reckon it is totally worth it!