I think solo is better.
Well this is my situation: I am RP, and I always do It with an Ed. We attack the same to the Boss (2 turns each one) but I also tank It while the ED sio me. Our loot? AI normaly get like 7 different things, he only gets a green gem, always.
The times he does It alone he gets as I do, over 7 different items...
So I guess that, at least, to do a little profit It is better to do It solo