I think it's much easier solo. The big advantage of boxing, in my opinion, is the chance to get more drops in inferno, in about the same time as playing one character.

From what I can tell, most people farm solo or with one other player, because the way things scale.

So when you add another player, the monster's hitpoints more than double.

In Inferno (it scales less for normal/nightmare/hell):
1 player = 100% monster hitpoints
2 players = 210%
3 players = 320%
4 players = 430%

So a 200k hp monster turns into a 860k hp monster in 4 player mode.