I voted "I've nursed twins and a singleton. The twins were a little harder, but not by much", simply because it requires more "wrangling" of bodies. Also, mine boys were born early, so they had to learn how to nurse.
On the other hand, it is WAY more entertaining sometimes than nursing one. I was sitting on the floor today with my shirt up, all kicked back, and the boys were snacking and giggling, and I asked my husband if when we first met he ever thought this is what he would see in his living room!
The hand holding and talking and smiling and giggling between the two balances out the other inconveniences!